Make parser unambiguous

[jakobnissen]

See BioJulia/Automa.jl#48

The parsers in GeneticVariation.jl are ambiguous, and will break if BioJulia/Automa.jl#49 gets pushed. Importantly, it means that even if the PR is not pushed, it's currently possible to create inputs where the parser will take unexpected actions.

I can take a look at the parser some other day, if necessary. If you want to play around with it yourself, get the version of Automa in the PR linked above, then try it out.

[TransGirlCodes]

The VCF parser in this package should be moved to a VCF.jl in line with the other parsers like FASTX, so depending on time it may be better to fix it as part of that work, unless it's urgent.

[jakobnissen]

Not urgent, I don't use VCF files in my work.