add additional .vtt data

Mark WebVTT files as:

 * having the default charset `UTF-8`:

   "A WebVTT file must consist of a WebVTT file body encoded as UTF-8"

   (from http://dev.w3.org/html5/webvtt/#h2_syntax)

 * compressible - they are text files, therefore, they compress
   quite nicely.

closes #9
add note on .woff compression
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