Add support for partial tests, and a test watcher

[antdking]

Implement pytest-watch and pytest-testmon to support watching tests.

pytest-watch monitors filesystem events to trigger tests to run whenever a change is done.

pytest-testmon utilises Coverage to tell which tests are affected by a change, and targets them.
Together, you end up with a really quick cycle for development.

example implementation: https://github.com/FundingOptions/Mallard/pull/7

depends on #9