Added a '-k' flag for preserving information for a genotype filter.

Instead of simply replacing the entry with a '.', the genotype section
is replaced with a './.', but preserves the rest of the information for
that entry.  

A filtered entry for '-g "DP > 5"' WITHOUT '-k' flag would look like:
.

A filtered entry for '-g "DP > 5" -k' would look like:
./.:30.5931:2:2:79:0:0:0,-0.563566,-3.78195