Add support for WebVTT

[joksas]

I use https://www.iana.org/assignments/media-types/text/vtt for magic numbers, as suggested here.

I have very little experience with file signatures, so maybe that's stupid but I've excluded the fifth and last byte sequences in the link because they end with EOF; or is there a way to represent this using the existing functions?

[gabriel-vasile]

I think you should add the checks for EOF too because IANA recommends so.
Unfortunately prefix cannot handle EOF so you'll have to write Vtt function yourself, similar to zstd for example. You can use bytes.HasPrefix for non-EOF cases and bytes.Equal for EOF cases.

When it comes to MIME types and magic numbers IANA is THE authority, so I try to follow them as much as possible.

[coveralls]

Coverage remained the same at 96.407% when pulling ce457ac on joksas:vtt into e7d7726 on gabriel-vasile:master.

[gabriel-vasile]

Hi, sorry for the late reply. I left two suggestions.

[joksas]

Hi,

Thank you for the comments. Should be fine now, but please let me know if I need to change anything else.