Another plugin system

[garthk]

Like @Enderboi's #35, but with:

• Added support for directory modules as well as single files
• Switched to initialise functions to match the rest of the framework
• Excluded plugins named initialise to avoid a namespace clash after import plugins
• Took out the global parameter: the plugin system always runs at boot
• Added implicit parameters to disable plugins
• Some documentation
• Appropriate changes to the deployment scripts

Convo here or there fine.

[garthk]

Sorry about that; took out the dependency on #34.

[garthk]

I've added another example plugin to automatically chase some colours across the pixels at boot time.