robustness fix to _parse_extinf

[deutrino]

function now handles titles with commas, and coerces unset durations to 0

[jbochi]

Thanks for the pull-request, but I think we should not merge it as it is.

1. I think the solution for titles with commas is ok, but we should add a test case.

2. The spec does not mention how blank durations should be handled, but I think we should set it to None internally, so we can dump it as blank and not 0. Tests would be nice two.

[feuvan]

Just follow the same way using ATTRIBUTELISTPATTERN should be ok.
see _parse_stream_inf and _parse_key

[leandromoreira]

@jbochi @feuvan and @gordon-morehouse is it still valid to apply or should we wait for test case?