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.
The use of an error throw in order to allow people to just write `this.skip()` without a return is interesting. Though I know some people feel bad about using exceptions as flow control (“they’re supposed to be exceptional!”), it seems like a super elegant solution since we already have an error handler set up to handle test flow control.