Compress WebVTT files (`.vtt`)

* WebVTT files are basically text files, therefore, they compress
  quite nicely.

* Browsers that have support for WebVTT don't encounter any problems
  when the `.vtt` files are being served compressed.

Ref: http://dev.w3.org/html5/webvtt/
     http://tools.ietf.org/html/draft-pantos-http-live-streaming-13
Add configs for text files (`.txt`)

Prevent Apache from sending the HTTP headers intended only for
HTML documents when serving text files.
Add all image types to the cache busting list

Add `cur`, `ico`, `svg`, `svgz` and `webp` to the filename-based
cache busting list.