fix END in non VCF files with ref and alt columns

v0.1.1

• set number=1 when op=min see #40
• fix when we get -0 converted to just '-' (#42)
• when an annotation bed file has a header with 'ref' or 'reference' and 'alt' or 'alternate',
  then match on the ref and alt values defined in those columns (not just on position as for bed).
• (vcfgo) allow double-digit number in header thanks @chapmanb
• correct END position when using ref and alt from a file with only a position (infer end as start + len(ref)) [thanks Tomas].
• about 15% performance improvement mostly due to go1.8