pluggable types: BAM

[michaeltneylon]

use pysam to support BAM type by reading into SAM format before diff. Make it an optional installation since it requires an extra dependency.

[michaeltneylon]

I did a small proof of this locally and it works fine, but I had not made it pluggable. This is the part I want to get working better, so we can do something like pip install pytest-cromwell[bam]

[jdidion]

The [bam] part is just a matter of adding to setup.py

extras_require={
  "bam": ["pysam"]
}

[jdidion]

Implemented by #10