Add support for controlling the value of `*print-length*'

This commit allows the user to avoid killing Emacs when accidentally
printing large data structures in the REPL. It accomplishes this by
managing the Clojure var `*print-length' which controls the number of
objects in a collection to print.

Introduces a customizable variable:

- `cider-repl-print-length': if set to an integer, it will be used to
  set the value of `*print-length*' for each new REPL session started.

Introduces an interactive function:

- `cider-repl-toggle-print-length-limiting': for toggling whether to
  limit the print length or not