Whitelist .vtt.
Whitelist .vtt.

Closes #77.
cnbin/rawgit,rgrove/rawgit,gnowxilef/rawgit,ddtxra/rawgit,ssddi456/rawgit,ssddi456/rawgit,rgrove/rawgit,cnbin/rawgit,ddtxra/rawgit,ssddi456/rawgit,ddtxra/rawgit,sereepap2029/rawgit,gnowxilef/rawgit,sereepap2029/rawgit,cnbin/rawgit,gnowxilef/rawgit,sereepap2029/rawgit