Add setting to only push to a certain application

Given this app is made for pushing commands to the terminal, we should be able to say 'always push to Iterm2'. That way, if we coincidentally had another application focused last, we won't accidentally push to that application.

I'm not sure how to go about this. osascript seemed pretty sluggish when I used it to check the name of the last focused application (hence why that feature isn't here).