Mime types: add type text/vtt for `.vtt` text track files
Mime types: add type text/vtt for `.vtt` text track files

The type is defined in
<https://www.iana.org/assignments/media-types/text/vtt>.

kazu-yamamoto/wai,yesodweb/wai,kazu-yamamoto/wai,yesodweb/wai