Improve a bit the handling and display of clojure-mode-version.

`clojure-mode-version` is better off being a variable and it's nice
to have an interactive function to display it. Most modes add a reference
to a function such as `clojure-mode-display-version` in their menus.
Merge pull request #130 from bbatsov/version

Improve a bit the handling and display of clojure-mode-version.