Add ability for "watch" behavior

[chuck-flowers]

Skaf should be able to run in "watch" mode where it updates test files as the user makes changes to the source files in real time. Similar to watch tools that automatically rebuild and deploy as source code is changed.

[chuck-flowers]

This functionality might be better supplied by a separate 'watch' tool that will invoke skaf rather than having the skaf tool implement the watch behavior itself.