[feature] Add pre/post commands to custom jobs

Tangentially related to running multiple commands (see #165) there is another issue that is tricky to work around: needing setup and/or teardown actions before running a test.

I have a project using Rust and Maturin to build a Python module, which can then be tested with Pytest. Just running `pytest` with some `env` vars set is okay to run tests but there are two issues:

* before running tests at all, some initial setup of a venv needs to be done. This setup should *NOT* be run every time the watch triggers a test run update, only once.
* the only thing the watch accomplishes is catching changes to the test, with the build step involved (in my case `maturin develop` also needs to be triggered if the Rust code changes.

The second of these can be handled with two work-arounds (either putting it in a script to run before pytest every time (slow but simple) or running it in a watch on the side in addition to bacon (faster but more complex)), but the first one basically means `bacon` can't be launched directly, it needs to be launched from a wrapper that does the setup for it.

I would imagine it is possible to have `bacon` handle both of these cases as well many more by introducing one or two new features:

* A pre/post (setup/teardown) command could be configured in addition to `env` keys that would be executed once before (or after) the test watch loop is entered.
* A during (background) command could be launched out of sight that is expected to stay running while the bacon test interface is active, then killed off when it is done.