Proof-of-concept of macro-based refactoring

If [Poker](https://github.com/ctford/poker) has been `require`-ed, `crut`, `crtf`, and `crtl` perform simple unthreading, threading-first and threading-last refactorings on motions.

The basic idea is that vim-fireplace should delegate to Clojure macros via nREPL for the actual refactoring. Does this seem a reasonable approach?

Things that are missing:
* auto-requiring Poker
* extracting and inlining
* making the keybindings work more like `Eval`
* formatting what comes back