Added spawn default app module

[Bennett-Wendorf]

This adds a module that includes a function to store and then later spawn default applications for each group. A user can set a single key bind (I use mod + t) that when pressed will spawn the default application for the current group.

This is the initial implementation of this solution. It isn't quite clean enough for my liking, so I also wrote a more integrated version over on the main qtile repo here. My intention is to present both solutions and let the maintainers decide which solution better fits the project.