Remove the region nullification from the parser

Scripts can already get cues into the state that this tries to avoid,
and it seems harmless due to how the rendering algorithm works.

https://www.w3.org/Bugs/Public/show_bug.cgi?id=25948
Merge pull request #88 from foolip/rm-region-null

Remove the region nullification from the parser
Merge commit 'f0d4c6855f0facf9b75f8369c7e43c94e9028e2f' into revert-position-size-align

Conflicts:
	webvtt.html