Support -R/--regions-file and -T/--targets-file

[tomwhite]

Also

• Add complement flag to GenomicRanges
• Only use targets for filtering (refining), to be consistent with bcftools

The last one is a change to make vcztools more like bcftools, motivated by this line in the bcftools manual:

Both -r and -t options can be applied simultaneously: -r uses the index to jump to a region and -t discards positions which are not in the targets

Fixes #72