Add stop-prepl! and make figwheel optional

• Breaking: Figwheel is now an optional dependency so you'll need to add it to your own project.clj or deps.edn to use it as an environment. The exact dependencies are shown in the project's deps.edn and README. (#4)
• Updated all dependency versions, including ClojureScript.
• Add a stop-prepl! function that takes the options returned by start-prepl!, as requested in #6.
• Delete the prepl port file (if created by Propel) on stop-prepl! or process exit.

Fix REPL reading for some ClojureScript syntax

The REPL that Propel starts struggled with some CLJS things like #js ..., I've swapped it out so it should handle any valid Clojure or ClojureScript.

REPL into any prepl

I've re-implemented conjure.core/repl in terms of a remote-prepl, this means that any --env on the CLI will drop into a local CLI REPL, not just some of them as it was before.

You can also use --repl-only to skip starting a prepl, this allows you to REPL into any existing instance, such as a remote server.

clj -m propel.main --repl-only --port 8787
cljs.core=> (js/alert "Yay!")

Added lein figwheel support

The name says it all really, you can now pass lein-figwheel as an environment and have it start up figwheel from your project.clj, this addresses #2 🎉

Enjoy!

Initial release

Not much to see here, enjoy the tool!