Add a cider-print-level defcustom

Controls the depth of printing of a stacktrace cause's data.  Can be used to
prevent circular data structures from causing exceptions not to display.

Depends on https://github.com/clojure-emacs/cider-nrepl/pull/112
I was in two minds as to whether this would be useful elsewhere, such as in `cider-pprint-eval-last-sexp`.