Allow better control of processes used for stub generation

[cursive-ghost]

Reported by @eoliphan via email:

I'm using a feature of a lib (Malli + custom registry) that, for now at least, requires me to set a java system property. It not being set causes an exception to be thrown and that in turn blows up stub generation.

It is already possible to set the command line shortening for the stub process at File | Project Structure | Modules | | Clojure, but it should be possible to set JVM args and environment variables there too. The rest of the application execution parameters (working dir, main class etc) don't make sense for the stub processes.