Add "watch" mode to CLI options

[bmchristensen]

Some test runners, Jest for example, have CLI options to watch project files and run the tests on the detection of a change to those files. This improves workflow during strict TDD.

Although I may be mistaken, I haven't seen any setting or option to do this with green. I certainly think this would be a useful enhancement.

In a similar vein to Jest, from the CLI:
green --watch
green --watchAll

[CleanCut]

I haven't seen any setting or option to do this with green.

Correct. Green does not have this functionality. I wonder if there is some other utility that one could use to do this sort of thing. I would imagine there must be something out there where you could just do...

watch_stuff myproj 'test_*.py' -c "green ..."