add cs_long flag to mappy.Aligner.map, and a "-C" option to minimap2.py

[nickzoic]

Adds a cs_long flag to mappy.Aligner.map, to let Python code get the "long CS" format string.

# generate a short cs string
aligner.map(seq, cs=True)

# generate a long cs string
aligner.map(seq, cs_long=True)

Also adds a -C command line option to minimap2.py by way of example.