Allow for transparent / vibrant window theme

![image](https://user-images.githubusercontent.com/5620073/29191214-a775c87e-7e25-11e7-83e1-39eadbce7851.png)

Maybe it can be enabled with `{ transparent: true, vibrancy: 'dark' }` ?

https://github.com/electron/electron/pull/7898 — Vibrancy PR
https://electron.atom.io/docs/api/frameless-window/#transparent-window