change thrift_linter.py to use new register_options

Testing Done:
Take two. Previous change accidentally added "default=False" which was inappropriate and ended up breaking the logic that falls back to a value defined in the BUILD file. Now we explicitly default the flag to None and preserve the desired logic.

I ran the integration tests related to thrift linter, which I inadvertently broke in my last attempt at this:
  devpants goal test tests/python/pants_test/tasks:thrift_linter_integration

Reviewed at https://rbcommons.com/s/twitter/r/1225/