Simplify cue-settings API (Should land before #414/#413)

No longer exporting `webvtt_cue_set_setting_word` (`_word` replaced with
`_from_string` to be less nonsensicle). The reason for this is that it is not
something that should be particularly useful to an application (they would 
surely use `_set_setting` instead, if they really wanted that sort of 
functionality!)

Additionally, added better comments to `webvtt_cue_validate_set_settings`, and
removed currently nonsensical "return if status is PARSE_ERROR or BAD_CUE", 
because n'either situation is currently possible. Will re-add a similar
statement if necessary in the near future. (This may be necessary once #410 is
addressed, as it should afford more control over the behaviour of the parser)