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.