Add new binary frame

[onderweg]

Hi,

First: thanks for this nice library!

I would like to write a non-standard frame type (ETCO).
This is a binary frame type, so I suppose I cannot use ID3v2_Tag_set_text_frame.

I'm able to create the frame ID3v2_Frame struct manually, but I'm not able to add the frame to the frame list of the tag, because FrameList_add_frame() is private.
Is there any way I can add a frame, that is not text, comment or apic frame, but a binary frame, to the list of frames of a tag?
Or, would it be possible to make FrameList_add_frame non-private?

Thanks!

[larsbs]

@onderweg Hey! I'm not against making FrameList_add_frame non-private if it's required. Feel free to submit a PR with it (and a use case in the description) so I can review it and add it if all is good 👍