add "Previous message" command

ctrl+p, like nethack. Cycles through messages, also like nethack.

May want to add some sort of indicator of how many messages there have
been.

src/types/command.rs

@@ -1,11 +1,17 @@
 use super::Direction;
 use super::Direction::*;
 use termion::event::Key;
-use termion::event::Key::Char;
+use termion::event::Key::{Char, Ctrl};
 
 pub enum Command {
+    /// Quit the game
     Quit,
+
+    /// Move the character in a direction
     Move(Direction),
+
+    /// Display the previous message
+    PreviousMessage,
 }
 
 impl Command {
@@ -17,6 +23,7 @@ impl Command {
             Char('k') | Char('w') | Key::Up => Some(Move(Up)),
             Char('j') | Char('s') | Key::Down => Some(Move(Down)),
             Char('l') | Char('d') | Key::Right => Some(Move(Right)),
+            Ctrl('p') => Some(PreviousMessage),
             _ => None,
         }
     }