Add `clojure-find-def` for var definitions.

This is like `clojure-find-ns`, but returns the `def*` macro name and
name of the var.
Work around current syntax table for symbol names.

Replace `\sw` with `(\sw|\s_)`. Particularly in emacs regex notation,
this does not help readability. If/when the `\w` class is updated in
the syntax table, this commit can be reverted.
Merge pull request #238 from jeffvalk/master

Add `clojure-find-def` for var definitions.