YES/NO enumerated-string parsing returns string rather than bool

[davemevans]

There are a bunch of attributes of type enumerated-string with values "YES" (and sometimes "NO") (eg EXT-X-DATERANGE END-ON-NEXT).

The parser will currently return these as raw strings. Turns out this is a bit awkward to work with, and these values would be more straightforward to use if returned as bools (ie testing == "YES" rather than is True).

I can put together a patch to parse these attributes as bools, but it would be a breaking change.

What do people think?

[mauricioabreu]

@davemevans sounds good. Submit your PR, then we can discuss there. Thank you!