REQUEST: Support for New/Arbitrary Tag fields e.g. WhiteElo, and PNG {comments}

[raysmith619]

Probably an accessible hash in PGNGame, e.g tagHash name/value.
After some looking, I found __getitem__(), which handles access to tags, WHEN you know the tag. I could not find access to the list of tag names. This would be helpful when reading/writing arbitrary PNG files. Also it would be helpful to provide access (set/get) to PNG comments.

Note, in general bulletchess is great plus FAST.