Get up to speed with features provided by inf-clojure

[johnwalker]

Inf-clojure has many features that we aren't using yet. See:

https://github.com/clojure-emacs/inf-clojure/blob/master/inf-clojure.el#L66-L71

We still need to add

• inf-clojure-show-arglist
• inf-clojure-show-var-documentation
• inf-clojure-show-var-source
• inf-clojure-clear-repl-buffer