Save and restore window position and size

[skullydazed]

Currently we always open at the same size and centered on the screen. We should remember where the user places the window and open there instead. After #36 we'll also want to save and restore the window size.

Two possible solutions:

• https://www.npmjs.com/package/electron-window-state
• https://www.npmjs.com/package/electron-window-state-manager