Fixes #141 - Removed `line_is_relative` variable from cue settings

Removed `line_is_relative` and the union (if `line` is a percentage
greater than 100% it is considered a bad value, and therefore fits
perfectly into an int without requiring an unsigned value)

Ensure that `snap_to_lines` is set correctly when line cuesettings are
used
Merge pull request #141 from caitp/issue-141

Multiple state variables refer to same thing in webvtt_cue and WebVTT::Cue