Utility function to get a vorbis stream's length

I think there should be a function somewhere to get a vorbis stream's length in samples, much similar to this one in stb.

[TonalidadeHidrica]

@JackRedstonia I have already implemented in my PR :)

Great, please label that PR as closing this one as well.

[TonalidadeHidrica]

Done!

[est31]

I've originally implemented this in ogg-metadata, which is why I closed #25, but maybe one can add it to lewton natively as well.