Enable cider-ns-browser to show private vars via toggle

[MicahElliott]

Problem: It seems good practice to mark your non-public vars as ^:private (including fns from defn-). But then cider-ns-browser (a very useful view) doesn't show them. And it's still useful to document private vars.

Solution: Enable the private vars to be shown, with their docstrings. Could be a toggle command like a (for "show All") in the map (https://github.com/clojure-emacs/cider/blob/master/cider-browse-ns.el#L53).

Should probably still not show vars marked as ^:deprecated.

[bbatsov]

Should be a relatively simple change, so PRs welcome!