Add Rayon for parallel file checking

Now we're almost entirely IO bound, rather than having to wait to
read and search through files one at a time.

src/main.rs

@@ -10,6 +10,7 @@ fn main() {
 mod rack {
     use ::rack::config::Config;
     use ::rack::filematch::*;
+    use rayon::prelude::*;
     use std::fs::{self, DirEntry, File};
     use std::io;
     use std::io::prelude::*;
@@ -121,7 +122,7 @@ mod rack {
 
             let mut results = traversal
                 .entries
-                .iter()
+                .par_iter()
                 .filter_map(|e| self.check_file(&e.path).ok())
                 .filter(|mat| mat.matches.len() > 0)
                 .collect::<Vec<FileMatch>>();