Add demo functionality

[thewildmage]

Bare minimum is a button that lets you start the demo.

[sudermanjr]

Demo has configurable items such as:

  --brightness int       The brightness to run the demo at. Must be between min and max. (default 150)
  --count int            The number of loops to run the demo. (default 1)
  --delay int            The delay in ms of the demo program. (default 100)
  --gradient-count int   The number of steps in the gradient. (default 2048)

If we could control these that would be awesome-sauce