Add ability to specify files and directories to exclude when using watch mode

[nfachan]

Specific Maelstrom Program?
All test runners.

Problem Statement
The test runners have their own specific list of files and directories to exclude. However, if there is a file or directory that shouldn't cause a re-build and re-run, and that the test runner doesn't know about, you are stuck.

Proposed Solution
There should be a way to provide a list of prefixes to ignore, or a regular expression to ignore. This would presumably be specified as a configuration value.