WIP: Autolink URLs in blobs and diffs

Extracted from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4121.

Contains three attempts/approaches at autolinking:

• 134c848c
• 86c54979
• 798d3289

None of these have acceptable performance.

To do

• Make sure blob rendering performance doesn't suffer dramatically
• Don't run pre-processing pipeline, because this may break the highlighting

app/assets/stylesheets/highlight/dark.scss

@@ -185,6 +185,11 @@ $dark-il: #de935f;
     color: $dark-highlight-color !important;
   }
+  // Links to URLs, emails, or dependencies
+  .line a {
+    color: $dark-na;
+  }
+
   .hll { background-color: $dark-hll-bg; }
   .c { color: $dark-c; } /* Comment */
   .err { color: $dark-err; } /* Error */