Keep track of the block boundaries while decoding

[cmdcolin]

The block boundaries are needed in order to create uniqueId's for tabix, bam, etc type features

The way to do this is to modify our unzip code to either

a) Do a callback for each chunk to decode chunk features
b) Return a list of chunks and the canonical offsets

Then this change will be propagated to BAM and tabix codebases

[cmdcolin]

cc @rbuels @garrettjstevens

[cmdcolin]

Fixed by #21