EUnit plugin

Running `make tests` will now also run EUnit tests.

Users can set different `ERLC` and `EUNIT` options by providing the `EUNIT_ERLC_OPTS` and `EUNIT_OPTS` variables, respectively.
This is a fragile regex, relying on matching either the original supplied `EUNIT_OPTS` in which case simple shell match the strings at #L22, or hoping that the user supplied options are coincidentally in the right order without any duplicates.

I'd suggest either:

- drop this completely, does anybody use a different report dir?
- split out `EUNIT_REPORTDIR` or similar as a separate variable and then drop the regex completely.

BTW instead of echo ... you can use `$ENV{'EUNIT_OPTS'} directly in perl.