Add defaultX/defaultY options for setting default position of window

[rhysd]

This PR adds following options:

• defaultX: The X position that should be returned if no file exists yet. Defaults to 0.
• defaultY: The Y position that should be returned if no file exists yet. Defaults to 0.

Motivation of these options are that I want to specify default position of the window. This library specifies x: 0, y: 0 as default position, but default position of browser window is not (0, 0). The default position is center of screen.

These options accepts undefined because x and y options in BrowserWindow's constructor accepts undefined. When they are set to undefined, the window is aligned at center of the screen.

Note that this PR does not break current behavior because their default values are 0. I added test case for this and updated type definitions also.