Add ability to read BCF

[mbhall88]

As varifier uses pyfastaq to parse VCF it is unable to parse BCF format.

I know this is probably a big-ish change but I would suggest switching to using an API such as cyvcf2. This way the user doesn't have to convert all of their BCF files to VCF in order to use varifier. Additionally, cyvcf2 will likely give a decent speedup.

[mbhall88]

As a temporary workaround, you can convert with bcftools and pipe into stdin of varifier

bcftools view -O v test.bcf \
| varifier vcf_eval truth.fasta ref.fasta - out_dir

[martinghunt]

changing to cyvcf2 is a bigger change than I want to spend time doing. We could just have an open VCF function that pipes through bcftools if the file name ends with .bcf.