bgzf support

[Quantumplation]

bgzip files are an extension to gzip that takes advantage of the fact that you can have arbitrary data before and after the actual contents of the gzip file; it concatenates together a bunch of gzip files in 64kb chunks, and then uses an index file to record the offsets of the gzip headers. This lets you decompress subsets of the file if you know where to look, without having to decompress the whole file.

Currently, ouch interprets these files as a normal gzip file, and so only unpacks the first 64kb chunk.

It'd be great to get full bgzf support :)