Add support for additional text encodings.

This includes, but is not limited to, UTF-16, latin-1, GBK, EUC-JP and
Shift_JIS. (Courtesy of the `encoding_rs` crate.)

Specifically, this feature enables ripgrep to search files that are
encoded in an encoding other than UTF-8. The list of available encodings
is tied directly to what the `encoding_rs` crate supports, which is in
turn tied to the Encoding Standard. The full list of available encodings
can be found here: https://encoding.spec.whatwg.org/#concept-encoding-get

This pull request also introduces the notion that text encodings can be
automatically detected on a best effort basis. Currently, the only
support for this is checking for a UTF-16 bom. In all other cases, a
text encoding of `auto` (the default) implies a UTF-8 or ASCII
compatible source encoding. When a text encoding is otherwise specified,
it is unconditionally used for all files searched.

Since ripgrep's regex engine is fundamentally built on top of UTF-8,
this feature works by transcoding the files to be searched from their
source encoding to UTF-8. This transcoding only happens when:

1. `auto` is specified and a non-UTF-8 encoding is detected.
2. A specific encoding is given by end users (including UTF-8).

When transcoding occurs, errors are handled by automatically inserting
the Unicode replacement character. In this case, ripgrep's output is
guaranteed to be valid UTF-8 (excluding non-UTF-8 file paths, if they
are printed).

In all other cases, the source text is searched directly, which implies
an assumption that it is at least ASCII compatible, but where UTF-8 is
most useful. In this scenario, encoding errors are not detected. In this
case, ripgrep's output will match the input exactly, byte-for-byte.

This design may not be optimal in all cases, but it has some advantages:

1. In the happy path ("UTF-8 everywhere") remains happy. I have not been
   able to witness any performance regressions.
2. In the non-UTF-8 path, implementation complexity is kept relatively
   low. The cost here is transcoding itself. A potentially superior
   implementation might build decoding of any encoding into the regex
   engine itself. In particular, the fundamental problem with
   transcoding everything first is that literal optimizations are nearly
   negated.

Future work should entail improving the user experience. For example, we
might want to auto-detect more text encodings. A more elaborate UX
experience might permit end users to specify multiple text encodings,
although this seems hard to pull off in an ergonomic way.

Fixes #1

src/app.rs

@@ -96,6 +96,8 @@ fn app<F>(next_line_help: bool, doc: F) -> App<'static, 'static>
              .possible_values(&["never", "auto", "always", "ansi"]))
         .arg(flag("colors").value_name("SPEC")
              .takes_value(true).multiple(true).number_of_values(1))
+        .arg(flag("encoding").short("E").value_name("ENCODING")
+             .takes_value(true).number_of_values(1))
         .arg(flag("fixed-strings").short("F"))
         .arg(flag("glob").short("g")
              .takes_value(true).multiple(true).number_of_values(1)
@@ -251,6 +253,14 @@ lazy_static! {
               change the match color to magenta and the background color for \
               line numbers to yellow:\n\n\
               rg --colors 'match:fg:magenta' --colors 'line:bg:yellow' foo.");
+        doc!(h, "encoding",
+             "Specify the text encoding of files to search.",
+             "Specify the text encoding that ripgrep will use on all files \
+              searched. The default value is 'auto', which will cause ripgrep \
+              to do a best effort automatic detection of encoding on a \
+              per-file basis. Other supported values can be found in the list \
+              of labels here: \
+              https://encoding.spec.whatwg.org/#concept-encoding-get");
         doc!(h, "fixed-strings",
              "Treat the pattern as a literal string.",
              "Treat the pattern as a literal string instead of a regular \
@@ -335,9 +345,9 @@ lazy_static! {
               provided are searched. Empty pattern lines will match all input \
               lines, and the newline is not counted as part of the pattern.");
         doc!(h, "files-with-matches",
-             "Only show the path of each file with at least one match.");
+             "Only show the paths with at least one match.");
         doc!(h, "files-without-match",
-             "Only show the path of each file that contains zero matches.");
+             "Only show the paths that contains zero matches.");
         doc!(h, "with-filename",
              "Show file name for each match.",
              "Prefix each match with the file name that contains it. This is \