output: add --stats flag

This commit provides basic support for a --stats flag, which will print
various aggregate statistics about a search after all of the results
have been printed. This is mostly intended to support a similar feature
found in the Silver Searcher. Note though that we don't emit the total
bytes searched; this is a first pass at an implementation and we can
improve upon it later.

Closes #411, Closes #799

src/args.rs

@@ -79,7 +79,8 @@ pub struct Args {
     type_list: bool,
     types: Types,
     with_filename: bool,
-    search_zip_files: bool
+    search_zip_files: bool,
+    stats: bool
 }
 
 impl Args {
@@ -221,6 +222,12 @@ impl Args {
         self.max_count == Some(0)
     }
 
+
+    /// Returns whether ripgrep should track stats for this run
+    pub fn stats(&self) -> bool {
+        self.stats
+    }
+
     /// Create a new writer for single-threaded searching with color support.
     pub fn stdout(&self) -> termcolor::StandardStream {
         termcolor::StandardStream::stdout(self.color_choice)
@@ -411,7 +418,8 @@ impl<'a> ArgMatches<'a> {
             type_list: self.is_present("type-list"),
             types: self.types()?,
             with_filename: with_filename,
-            search_zip_files: self.is_present("search-zip")
+            search_zip_files: self.is_present("search-zip"),
+            stats: self.stats()
         };
         if args.mmap {
             debug!("will try to use memory maps");
@@ -825,6 +833,19 @@ impl<'a> ArgMatches<'a> {
         }
     }
 
+    /// Returns whether status should be tracked for this run of ripgrep
+
+    /// This is automatically disabled if we're asked to only list the
+    /// files that wil be searched, files with matches or files
+    /// without matches.
+    fn stats(&self) -> bool {
+        if self.is_present("files-with-matches") ||
+           self.is_present("files-without-match") {
+               return false;
+        }
+        self.is_present("stats")
+    }
+
     /// Returns the approximate number of threads that ripgrep should use.
     fn threads(&self) -> Result<usize> {
         if self.is_present("sort-files") {