Updates to the Command API to support implementation

[dualspiral]

The big thing here is the addition of a "CommandRegistrar". This is meant to be a lightweight interface that tries to give external command frameworks (Aikar's ACF, Katrix's Scammander, Faith's Butler, amongst others) a simple way to add their commands to Sponge. The idea of registrars was originally an implementation one - if you look at that code, you'll see that the vanilla brig system is separated from the Sponge one - it takes little work to add support for arbitary registrars, so why not add them?

The Javadocs should provide a fair amount of explanation. There will be more for client completions, I'm sure - though enabling that is the thing that is missing right now. That'll come later - it'll likely mimic Brig's suggestion provider with a tree like format to build the command elements up with.

This is simply for review of code style and Javadocs.

[ItsDoot]

So effectively, CommandRegistrar is similar to a Dispatcher + CatalogType?

[dualspiral]

I guess, though it doesn't really need to be a dispatcher so much. A registrar could conceivebly be a command executor - I don't put a restriction on it.

Note that there are a couple of classes that don't have JDs. That's a mistake on my part, some API elements have made it through that were nowhere near ready. I won't be merging tonight as it is, so pick on anything and I'll review over the next couple of days.