Switch from .rgignore to .ignore.

But don't actually remove support for .rgignore until the next semver
bump.

Note that this puts us in line with the silver searcher:
https://github.com/ggreer/the_silver_searcher/pull/974

Fixes #40

src/args.rs

@@ -137,7 +137,7 @@ Less common options:
         Never use memory maps, even when they might be faster.
 
     --no-ignore
-        Don't respect ignore files (.gitignore, .rgignore, etc.)
+        Don't respect ignore files (.gitignore, .ignore, etc.)
         This implies --no-ignore-parent.
 
     --no-ignore-parent

src/gitignore.rs

@@ -9,7 +9,7 @@ The motivation for this submodule is performance and portability:
 2. We could shell out to a `git` sub-command like ls-files or status, but it
    seems better to not rely on the existence of external programs for a search
    tool. Besides, we need to implement this logic anyway to support things like
-   an .rgignore file.
+   an .ignore file.
 
 The key implementation detail here is that a single gitignore file is compiled
 into a single RegexSet, which can be used to report which globs match a

src/ignore.rs

@@ -5,7 +5,7 @@ whether a *single* file path should be searched or not.
 In general, there are two ways to ignore a particular file:
 
 1. Specify an ignore rule in some "global" configuration, such as a
-   $HOME/.rgignore or on the command line.
+   $HOME/.ignore or on the command line.
 2. A specific ignore file (like .gitignore) found during directory traversal.
 
 The `IgnoreDir` type handles ignore patterns for any one particular directory
@@ -24,6 +24,7 @@ use types::Types;
 
 const IGNORE_NAMES: &'static [&'static str] = &[
     ".gitignore",
+    ".ignore",
     ".rgignore",
 ];