Infrastructure for debugging natures and plugins.

* Can provide suggestions for how to define a plugin given a context (a loaded Project in practice).
* When a user requests an undefined key at the command line, can indicate whether any (deactivated) plugins provide the key.

TODO:
* Hook up to the key parser
* Implement 'help <plugin>'
* Determine how to best provide the context (the current project is often an aggregating root, which is not typically a useful context)