Allow plugins to store things that get checkpointed

[nathankleyn]

Building on the work proposed in #15, it's a natural extension to want to allow plugins to store things as well that only get saved properly when the shanty checkpoint task is called. As such, this should be implemented as normal with events so that plugin can hook into this process and persist config.

An example is the SBT plugin. It has to call out to SBT to check the config of all projects and check if they're changed. This is quite expensive to do. It would be great to be able to save the output of this, and only call it if anything has changed. We'd want to save this only when shanty checkpoint is called.