ripgrep: add --pre flag

The preprocessor flag accepts a command program and executes this
program for every input file that is searched. Instead of searching the
file directly, ripgrep will instead search the stdout contents of the
program.

Closes #978, Closes #981

src/args.rs

@@ -80,6 +80,7 @@ pub struct Args {
     types: Types,
     with_filename: bool,
     search_zip_files: bool,
+    preprocessor: Option<PathBuf>,
     stats: bool
 }
 
@@ -288,6 +289,7 @@ impl Args {
             .quiet(self.quiet)
             .text(self.text)
             .search_zip_files(self.search_zip_files)
+            .preprocessor(self.preprocessor.clone())
             .build()
     }
 
@@ -429,6 +431,7 @@ impl<'a> ArgMatches<'a> {
             types: self.types()?,
             with_filename: with_filename,
             search_zip_files: self.is_present("search-zip"),
+            preprocessor: self.preprocessor(),
             stats: self.stats()
         };
         if args.mmap {
@@ -722,6 +725,19 @@ impl<'a> ArgMatches<'a> {
         }
     }
 
+    /// Returns the preprocessor command
+    fn preprocessor(&self) -> Option<PathBuf> {
+        if let Some(path) = self.value_of_os("pre") {
+            if path.is_empty() {
+                None
+            } else {
+                Some(Path::new(path).to_path_buf())
+            }
+        } else {
+            None
+        }
+    }
+
     /// Returns the unescaped path separator in UTF-8 bytes.
     fn path_separator(&self) -> Result<Option<u8>> {
         match self.value_of_lossy("path-separator") {