New formula: GNU Datamash

Signed-off-by: Assaf Gordon <assafgordon@gmail.com>
@mikemcquaid : during installation, the "make check" runs a battery of tests (see https://github.com/agordon/datamash/blob/master/tests/datamash-tests.pl#L255 and https://github.com/agordon/datamash/blob/master/tests/datamash-stats.pl#L250 ) - so that once "make check" is successfully completed (and before "make install" is run) - one can be sure the binary file is compiled correctly and produces correct results.

For this reason, in the HomeBrew "test" step, I only if the binary can be executed - I already know it produces correct results.