Suggestion: Run affected tests when sources change

[novemberborn]

Currently when sources change all tests are rerun. It should be possible to track which tests depend on which sources. If a source changes, determine which tests depended on that source. Rerunning just those tests should make for a better --watch experience.

[vadimdemedes]

There's an issue for it already - #115. Closing this in favor of the original one.

[novemberborn]

Here I was thinking I had original ideas…