Require repl / don't trigger a launch of a new JVM

I'm currently using this locally to only carry out highlighting when it has an nrepl to connect to.

Fireplace will happily launch a JVM for you. But in my current work project, that takes ~20 seconds to start hanging the editor in the meantime..!

With this change, I can always open a file instantly, then at some point later launch a repl, then `:e` to get the highlighting.