Many bugs in vcfdistance

This fixes various bugs in vcfdistance's handling of edge cases of 0-3 variants as well as correcting a doubly-added tag value and a minor bit of documentation.

Automated tests are added to demonstrate what's broken and ensure that it stays fixed.  You can run tests with `make test`.  They require perl + the IPC::Run3 module from CPAN.  This module is the only dependency not shipped with core Perl.