Allow reading of (b)gzip-compressed VCF files

Astalavista can read gzip-compressed GTF files, but not (b)gzip-compressed VCF files. (Note that bgzip-compressed files are compatible with normal gzip files.)

Given that nowadays it is common for variant data to be provided as (tabix-indexed) bgzipped VCF files, and given the large sizes of genomic scale variant files, it would be convenient if Astalavista could read them directly rather than requiring them to be decompressed.