[WIP] Initial cut of Go support

This is a placeholder for Go support in Neovim. Marking as `WIP` but I would appreciate any comments/feedback now.

I have put some [overview notes into a Wiki](https://github.com/myitcv/neovim/wiki/Overview-of-writing-and-using-Go-packages-with-Neovim) that explain the proposed approach for Go. This is a good place to start. Please feel free to ask questions or provide feedback here however.

Thoughts/feedback/whatever graciously accepted.

Yes, this should be `rplugin`. I've also just spotted `s:RegistrationCommands` which I hadn't seen before. Looks like this could be used for the discovery step I've been looking for (i.e. does away with the need for the plugin manifest file)