Add macro expansion

Bind [me to full macro expansion of the form under the cursor (using
clojure.core/macroexpand), and bind ]me to a partial macro expansion
of the form under the cursor (using clojure.core/macroexpand-1).

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.

Personally, I associate expanding macros with getting the source or
docstring of a fn, so I put it in the documentation section of the help,
though it would also make sense to me in the evaluation section.