Support for writing Neovim plugins in Rust.

This crate doesn't actually do anything yet except open up a session that can communicate over TCP, Unix socket, stdio, or a child process, but in the near future it will provide full support for communicating with Neovim.

There's a lot that needs to stabilize before this will become possible, including Neovim's plugin API, Rust itself, and the msgpack crate which will be used for serialization.

Development will attempt to mimick that of the Python client.