WebVTT reader does not handle files with invalid signatures according to the specification

[palemieux]

The WebVTT reader should follow the algorithm at:

https://www.w3.org/TR/webvtt1/#file-parsing

Currently it cannot, for example, handle empty files or detect bad files that start with WEBVTTSSSS.

See #534 (comment)

[palemieux]

Closed by bcf311a