Add grep functionality

This PR adds `grep` and `listTests` commands to Intern. `grep` allows tests to be filtered by test ID. `listTests` lists all the tests that Intern will run, post filtering if `grep` is used. `grep` works from the command line and may be set in the test config file. `listTests` only works from the command line. See #123.
violate style convention, negative variables. also, this feels like a gross thing to expose as a part of the API…