[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.

Should be `rplugin/` I think. See:

- https://github.com/neovim/neovim/pull/1515
- https://github.com/neovim/neovim/blob/2c29b20af767bd7ebebeb41da14df4d66a19d5a0/runtime/doc/remote_plugin.txt#L97
- https://github.com/neovim/neovim/blob/bcab2905d83c4b6c0def50462752b1beb5548f10/runtime/autoload/remote/host.vim#L123