cursewords should (optionally?) store a settings config file

[thisisparker]

As of v1, cursewords leaves all state considerations to the .puz files it opens. That covers a lot, but is likely too limiting for future feature development. A future version, maybe on the v2 branch, should save a settings file, which could:

• store user-customized key mappings, including for hotkeys and navigation, addressing Quit shortcut conflicts with Ubuntu default 'close application' hotkey #14
• open the door for color customization, including possibly "themes", addressing Use color for highlighted word #10
• provide a better home for upcoming features that would be passed as flags, such as disabling the timer

I want to think about the best way to store these settings (and more) in a way that is portable, robust, and can be operated both with or without direct user input.