Add run-command similar to compile / run-test

[mattiasb]

In projectile there are keybindings for compile (C-c p c) and run-test (C-c p P).

For some projects it makes sense to have a default run-command that would execute the main compilation artifact of the project (as an async-shell-command I guess).

I don't know really how this should be implemented. Maybe the support code for different projects should just add a default run command to the history of async-shell-command (if that's even possible) or a dedicated run-command should be added to projectile.

I hope this makes sense. :)

Thanks for an awesome project!

[mattiasb]

Cool!