Added subreddit variable.

src/interpolate.js

@@ -43,6 +43,7 @@ function interpolate(pattern, user, post, item) {
             $postUser: post.user || user.user,
             $postDate: dateFns.format(post.datetime, dateFormat),
             $postIndex: post.index + config.library.indexOffset,
+            $subreddit: post.subreddit,
             $host: post.host.label
         });