Hotkey window similar to iTerm2

[mjclarke94]

iTerm2 has an incredibly useful feature (which I believe originated with some flavour of linux terminal) where a hotkey can be assigned to peek and then hide a persistent terminal window. A very common workflow is to be alternating between reading documentation in a browser and then doing something, and iTerm enables you to this cleanly if that something happens to be in a terminal.

Switching this round so that the browser window can be shown/hidden with a hotkey opens this workflow up to a much wider set of use cases, and is particularly useful when screen real estate is at a premium.