Plugin dependency resolution

[pironic]

This is to expand on #22 but will resolve the dependencies when loading or unloading any plugin.

If a plugin is loaded that depends on another, we will attempt to load that plugin too. If the loading of any dependency fails, we'll fail the whole load.

If a plugin that is unloaded depends on another, we'll unload each before the dependent.

[ylt]

Mostly implemented except for the failing, which will probably be implemented tomorrow