Don't use lexer to read timestamps.

The goal is to simplify and eventually remove the lexer.

webvtt_collect_timings_and_settings no longer relies on the lexer
for parsing integers, percentages, or timestamps, nor any of the
cuesetting keywords/special values. Therefore a significant chunk
of the lexer is now deletable. Horray.
Merge pull request #414 from caitp/issue-414

Don't use lexer to scan integers, percentages, or timestamps (BLOCKS #413)