Add configuration file evaluation

[sdilts]

In Stumpwm, users are able to have initialization code ran at startup so they can customize the WM's behavior. We need the same thing in Mahogany.

Requirements for a complete implementation:

• [ ] config files should follow the XDG base directory specification
• [ ] There should be a command line flag to startup Mahogany without this file.
• [ ] Enabling / disabling evaluation could be a compile time option.
• [ ] Errors during evaluation should not cause Mahogany to crash or shutdown.

Like with defcommand, not all of this needs to be implemented at once.