Add a conformance section.

Closes https://www.w3.org/Bugs/Public/show_bug.cgi?id=22591
Define WebVTT cue identifiers as unique.

Closes https://www.w3.org/Bugs/Public/show_bug.cgi?id=24674.
WebVTT region identifier must also be unique.
Merge pull request #113 from silviapfeiffer/uniqueIdentifier

Unique identifier
Merge pull request #115 from silviapfeiffer/conformanceSection

Add a conformance section.
Run format.py

The rendered output has been manually verified to be identical.
Manually indent </script> and </style>

format.py isn't smart enough to get these right.