Create WebVTTParsers with windows instead.

This way we don't have to keep passing in a window object
with every call to parse() or flush(), as in the case when
regions will be added in the future.
Bulk updates to the README file.

A lot of the recent changes didn't make their way into the
README. This fixes that.
Create VTTRegions directly.
Merge pull request #95 from RickEyre/vttregion

Create regions directly