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/app.rs

@@ -559,6 +559,7 @@ pub fn all_args_and_flags() -> Vec<RGArg> {
     flag_search_zip(&mut args);
     flag_smart_case(&mut args);
     flag_sort_files(&mut args);
+    flag_stats(&mut args);
     flag_text(&mut args);
     flag_threads(&mut args);
     flag_type(&mut args);
@@ -1488,6 +1489,25 @@ This flag can be disabled with --no-sort-files.
     args.push(arg);
 }
 
+fn flag_stats(args: &mut Vec<RGArg>) {
+    const SHORT: &str = "Print statistics about this ripgrep search.";
+    const LONG: &str = long!("\
+Print aggregate statistics about this ripgrep search. When this flag is
+present, ripgrep will print the following stats to stdout at the end of the
+search: number of matched lines, number of files with matches, number of files
+searched, and the time taken for the entire search to complete.
+
+This set of aggregate statistics may expand over time.
+
+Note that this flag has no effect if --files, --files-with-matches or
+--files-without-match is passed.");
+
+    let arg = RGArg::switch("stats")
+        .help(SHORT).long_help(LONG);
+
+    args.push(arg);
+}
+
 fn flag_text(args: &mut Vec<RGArg>) {
     const SHORT: &str = "Search binary files as if they were text.";
     const LONG: &str = long!("\