Add support for prompt widgets

[alberdingk-thijm]

I've just uploaded a crate to crates.io which contains some custom widgets I wanted for a Conrod-based project.

These widgets are for displaying a prompt to the screen along similar lines to your standard GUI prompt where a message is shown to the user, who then clicks a button to dismiss it or respond to it. The current design is very simple, but the general idea seemed like something more people might find useful, so I figured I'd upload it.

If there's interest in integrating the crate into the rest of conrod, I'd be happy to do so, whether as it currently exists or in a more robust form. I also saw on #1241 that there is some discussion of splitting off non-primitive widgets into another conrod_widgets crate; this work would fit there as well.

[alberdingk-thijm]

A working example has now been provided in version 0.1.1.