make 'Got' and 'Expected' column headers configurable

I'm using eq_or_diff to compare the structure of two database tables in a test, which for Necessary Reasons need to be identical. Having column headers 'Got' and 'Expected' isn't very helpful because it's not
immediately obvious which is which - after all, I've got them both.

So I've monkey-patched to make them configurable.

Patch attached. Includes doco but no tests because I'm very very lazy. Sorry about that :-)