Add option to start the application minimized

[cquike]

It would be useful if there is an option to tell mirage that at startup it should minimize itself in the icon tray. That's useful when logging in your computer and a number of applications are started by the desktop manager. You don´t want all these applications to clutter the desktop popping up windows, but rather that they start minimized.

[MRAAGH]

Use the --start-in-tray flag. I'm uncertain whether this works in Mirage, but it definitely works in Moment

[MRAAGH]

Duplicate of #164