Merge `vcf2fasta.py` and `vcf2nexus.py`

[J-Moravec]

It looks like vcf2nexus.py is just a copy-paste of vcf2fasta.py with a slightly different writer (to nexus instead of fasta).

At least it was before further enhancements of vcf2fasta.py, which is causing the code to diverge and vcf2nexus.py lag behind.

There seems absolutely no reason why these should be separate files instead of a simple option --format fas/nex/xml.

This should be done rather early, but likely after the issues in #5 are solved to prevent further duplication in codebase.