Add support for variables in eldoc

[bbatsov]

In Emacs Lisp even variables have eldoc:

Adding this to cider should be easy.

[bbatsov]

Btw, @cskksc as you've been doing some eldoc-related work lately you might eventually take a look at this as well. Basically we just need to make eldoc consider the symbol-at-point in addition to the first symbol of the list at point (which we currently do). So in a form like (map inc ...) if your cursor is over inc you'd see its docstring and when you move forward you'd see once again the docstring for map.

[ckoparkar]

Yeah. Will do. :-)