Update of vcf2txt.pl

vcf2txt.pl now is transparent to VCF files from single sample or paired,
annotated using SNFEff v3 or v4, and whether is amplicon awareness
calling.  Each scenario will produce corresponding text format for
downstream processing.

Notice -f and -F definitions were swapped for consistency with VarDict
and var2vcf_valid.pl script.  -a is now dropped as it'll automatically
detect whether it's amplicon awareness calling in VCF file.