disable AppNap if supported

That's a quick and dirty patch which may not be suitable for merging as is but could be a start :)

My use case is that I have my terminal in the background with an utility running in it which is monitoring the disk and run the tests when a file is changed, with the latest release I get a really annoying behavior when AppNap kicks in: the terminal takes more time to refresh and show me the test results.

that may be an edge case but I never want AppNap to be triggered for iTerm2 but maybe it could be implemented as an option for a more general use.