First draft of external test plan file

This commit shows how an external test plan can be used to define the
test scheduler rules without needing to customize the Makefile.PL or
Build.PL invocation of TAP::Harness.  It would also thus work seemlessly
with prove.

This has not yet been tested.  There is no validation of the contents
of the test file (though any existing rules validation in
TAP::Harness::Scheduler would of course apply).

I'm posting this as a PR to get feedback and solicit help implementing tests.