Allow theme or plugin creators to take advantage of vibrancy on macOS

[blakek]

Hey! Just as an enhancement, it would be nice if theme or plugin creators could take advantage of vibrancy on platforms that support it.

Electron doumentation for vibrancy

I see a couple ways for implementing this:

1. a new field in the theme's zazu.json (theme would probably need to be loaded earlier??)
2. create a new plugin type (e.g. middleware) that can modify the app window

I think the second option could be really interesting. You could allow plugins to fire on certain events including lifecycle events such as when the window is about to be created.

Thanks all!

[bayleedev]

I'm unfamiliar with vibrancy, what does setting it do?

[rodvilla]

@blainesch It gives a kinda-blurry background to the window. I.E.: