Fixes #279 - Correct declaration/definition of webvtt_string_is_equal

Added 'const' attributes to string parameters in webvtt_string_is_equal.
Additionally, properly casting parameter 2 when comparing against a literal
string, in order to prevent the warning from being thrown.
Fixes #279 - Remove unnecessary const cast
Merge pull request #279 from caitp/issue-279

[warning] webvtt_string_is_equal discards 'const' qualifier when using literal