Aligner::Cigar(string) constructor

[Peter]

A constructor that can parse the output created by the ostream operator.

[Peter]

In 4003:

Add a constructor Aligner::Cigar(string) (closes #962)

Add a function cigar_table, which is a wrapper around bam_cigar_table
from htslib, translating operation characters in a CIGAR such as 'M'
or 'I' into the value as they are stored in a Cigar object (or a bam
entry). Function, bam_cigar_table, not available in older versions of
htslib or when building --without-htslib, so implement our own version
and decide at configure time whether using homebrew or call htslib.