Added support for pausing and restarting Clojurescript REPL's.

The keyword :cljs/pause will pause the active Clojurescript REPL and the function unpause-cljs-repl will restart the Clojurescript REPL.

I also added a new nrepl op: piggieback-repl-info which gives information about which REPL is active. This operation could be used by an nrepl client such as Cider to determine which REPL is active and then eval the appropriate command to switch REPL's using a keyboard shortcut.