Introduce a positionAlign value of "auto"

"auto" is the default positionAlign setting
It depends on the "align" setting.
Introduce "computed text position align"

allows removing of positionSetAlign in the rendering section
Merge pull request #98 from silvia/positionAlignAuto

Introduce a positionAlign value of "auto"

Conflicts:
	webvtt.html
Merge pull request #99 from silvia/positionAlignComputed

Introduce "computed text position align"

Conflicts:
	webvtt.html
Merge commit 'b4abad20c8b21f70e396b170cee5fdb25fd3c1ad' into revert-position-size-align

Conflicts:
	webvtt.html
Merge commit 'bfb16dfd234088aa33504f0eff3b4b42439a9249' into revert-position-size-align

Conflicts:
	webvtt.html