Support to async runs of scripts upon typing

[moretticb]

Script runs implemented in #53 are synchronous, and hence blocking. Support to async calls would be a nice feature to have to improve user experience.

[moretticb]

I'm not sure how to do it, as I'm not experienced in obj-c. An idea I had is to:

• Run a script in the background after a pause, say, two seconds after last keystroke.
• Kill any instance of the same script if user continues typing (at some point the pause will happen again, and hence another run)
• Once a script finishes running, an output can be written to some temp file and a signal be sent to choose.
• Have choose listen to process signals. Once it intercepts a signal, it reads the output from a temp file and outputs it