hammerspoon: Add shortcut for setting up screencast environment
hammerspoon: Add shortcut for setting up screencast environment

This is a quick hack, but it positions windows suitable for recording a Vim
screencast. For now, I just open the URL to trigger it (ie. `open
hammerspoon://screencast`; no key binding).

Strange glitch I noticed during the last screencast: windows actually resized
midway through. I think I am getting the occasional spurious event, so I might
want to tweak this further to suspend the event-based resizing behavior during a
cast.

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