Add APL support

See https://github.com/ngn/apl#readme and its demo.
An effort by @plj541 to provide a similar REPL is available at: http://home.comcast.net/~paul.l.jackson/APL.js/

APL uses a character set of its own, so users may need to spend some additional effort configuring their keyboards, but I think APL would be a nice addition to repl.it anyway.