Add separate enums for lineAlign and positionAlign

https://www.w3.org/Bugs/Public/show_bug.cgi?id=26016
Merge pull request #109 from foolip/align-enums

Add separate enums for lineAlign and positionAlign
Merge commit 'd4bacf9148bbdd95753b7c3cd9fc3c4c3fbbdd6b' into revert-position-size-align

Conflicts:
	webvtt.html