Support adding descriptions to controls.

[by-robots]

Controls are now able to be given a custom description to provide better context when viewing Storybook.js documentation.

Adding a description works in the same way as adding a custom name for the control — it can be set when initializing the control or by calling the description method on the instance.

[by-robots]

This is really awesome work. A couple of suggestions and need for slightly more specs and this would be ready to go.

Thanks, @jonspalmer — I'll sort those changes shortly :)

[jonspalmer]

Great work sorry it took so long to merge this.