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.
I wonder if we should throw a known object instead of creating a new one every time (`var SKIP = {}`, `throw SKIP`, `if (error === SKIP)`), and/or put the skip message in the error message?