Macroexpand motions

Bind cm{motion} to full macro expansion of the form under the cursor
(using clojure.core/macroexpand), and bind c1m{motion} to partial macro
expansion of the form under the cursor (using clojure.core/macroexpand-1).

As per cp{motion}, the suffix "p" will apply the expansion to the form
under the cursor.

When exploring Clojure code I find it very useful to see what macros
are doing prior to evaluation. For example, it's much easier to understand
defn once you expand it and see it's really using def to bind a name to
an anonymous fn.

This is my second attempt at implementing macroexpansion for vim-fireplace following @tpope's help with my first pull request (#69).