Notify users when some command is not supported under ClojureScript

[bbatsov]

Right now often people experience weird issues when they try to run something supported only for Clojure in a ClojureScript connection. It'd be nice if we added some basic function that does sanity checking in such commands and informs the users that the command they won't to use is not yet supporting "ClojureScript". Back in the day we've added some check only for the debugger, but it's time to expand this.

This command should be pretty easy to implement and should be applied to common interactive commands.

Any volunteers to take a stab at it? @xiongtx @ckoparkar @dpsutton