Milestone yat 0.13

The major focus of this release is to add support for htslib (#794)

We encourage users to use htslib rather than its predecessor libbam (samtools 0.1.x). Currently, there is no plan to drop support for linking against libbam, but as the APIs are slightly different between libbam and htslib and therefore occasionally need separate implementations, optimising the libbam implementation will not be a priority.