repl: exports `Recoverable`

Allow REPL consumers to callback with a `Recoverable` error instance
and trigger multi-line REPL prompts.

Fixes: https://github.com/nodejs/node/issues/2939
PR-URL: https://github.com/nodejs/node/pull/3488


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>