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 how many objects to
print in each collection.

Introduces two variables:

- `cider-repl-print-length`: the desired value of `*print-length*`
- `cider-repl-limit-print-length`: whether to limit the print length or not.

Introduces a interactive function:

- `cider-repl-toggle-print-length-limiting`: for toggling whether to
  limit the print length or not