MLLT support

[ahti]

Would you consider adding a content type for reading and writing MLLT tags?

The quirky thing about this tag is probably that the reference entries contain integers that can be dynamically sized, up to 256 bit. Their combined size has to be a multiple of four, but it seems like the integers themselves could be anything from 1 to 255 bits long (or maybe even 0 to 256? I'm not quite sure how to interpret the tag docs).

So supporting this would probably mean a fair amount of bit-twiddling and depending on something like num-bigint.

A middle way could be to support the meta fields, but decode the references into a single blob that consumers can then further encode/decode themselves.

[polyfloyd]

I think it's possible to manually encode and decode these tags using raw Content::Unknown frames.

However, we would happily accept a patch to add proper support for this feature. :)

[ahti]

Yes, that is what I have done for now.

For anyone that needs MLLT, this is my implementation, which also generates the mllt table from mp3 frames. It has a fixed number of bits for size and the code isn't very pretty, but it works.

[polyfloyd]

MLLT support has been implemented in 0ea0a7f which will be released with 1.0