Allow setting of excludes and includes via text files

It'd be handy in some circumstances to be able to read the excludes (and includes) from text files. This would allow for dynamically determining which tests to skip or include (for whatever reason) before the tests are executed, or for pulling in a list of tests to exclude/include from an external source, etc. We're planning to use this at Cloudera for segregating "flaky" tests from our primary test runs (and vice versa). Patch attached, though I'm sure it could still use work.