Add a generic text-prompt system

Add a generic text-prompt system to the Game, and use it to prompt the
character for their name on startup. There's also a Promise type in
util, which is used for the result of the prompt.

src/main.rs

@@ -26,6 +26,7 @@ extern crate include_dir;
 extern crate nom;
 #[macro_use]
 extern crate matches;
+extern crate futures;
 
 #[macro_use]
 mod util;