count-errors.py and CIGAR M operator

[peterjc]

Currently count-errors.py depends on CIGAR = and X operations, and cannot handle the older but widely used M operator (match or mismatch).

Ideally the script could be extended to handle these (slower), but at least a warning would be useful.

Issue spotted while following @mw55309's tutorial https://github.com/mw55309/EG_MinION_2016 where this can be used for LAST but not bwa mem.