VCF parsing

[james-guevara]

Should write a simple VCF parser for this package:
https://en.wikipedia.org/wiki/Variant_Call_Format
https://samtools.github.io/hts-specs/VCFv4.2.pdf


This has already been implemented with HTSlib (and its Nim wrapper https://github.com/brentp/hts-nim). But it'd be nice to have a pure Nim implementation.

Here are also hts-specs for Tabix index file format:
https://samtools.github.io/hts-specs/tabix.pdf