make "spring test" support any number of arguments

This obsoletes #101.

`spring test` needs to accept multiple arguments if guard-minitest is to ever have a chance of supporting spring.  Specs are automatically pruned from the command-line with a warning, since loading specs will stop the unit tests from running.

It's also useful for it to support zero arguments, in which case it does the sensible thing and defaults to running all unit tests.  Additionally, this is consistent with the behaviour of `spring rspec`.