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

This commit allows the user to avoid freezing/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 of a collection to print.

Introduces a customizable variable:

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

Introduces an interactive function:

- `cider-repl-toggle-print-length-limiting`: toggles whether to enforce
  `cider-repl-print-length` or not.
Merge pull request #430 from ragnard/control-print-length

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