Embedded CUE Sheet Support

[G2-Games]

Unless I missed something, I did not see any mention of being able to read embedded CUE sheets using Lofty. Is this a feature you would consider adding?

I currently have no good way to test this feature, but I can provide a file that includes a sheet like that.

References:
FLAC Spec

[Serial-ATA]

This seems simple enough to parse, at least.

Since this would be stored in FlacFile, two things would need to happen first, though:

1. We need some way to indicate whether or not the caller is using read_from{_path} or Probe::read() since the cue block is unnecessary to parse in those cases.

2. Since lofty only works with tags, the writing is done in their context (eg. FlacFile::write_to simply calls VorbisComments::write_to(file)). There'd have to be some changes to allow for a different, format-specific field to be written at the same time as the tag.