Add demo eat command

demo/demo.rpy

@@ -1,2 +1,3 @@
 "Bob sat on the bench."
 "Bob" "Good morning!"
+eat "potato"

src/lib.rs

@@ -33,7 +33,10 @@ pub enum Command {
     Say {
         name: Option<String>,
         text: String,
-    }
+    },
+    Eat {
+        food: String,
+    },
 }
 
 use Command::*;
@@ -124,6 +127,9 @@ pub fn parse_file(file_path: &str) -> Vec<Command> {
                 name: Some(name.to_owned()),
                 text: text.to_owned(),
             },
+            [Keyword(keyword), Str(food)] if keyword.eq("eat") => Eat {
+                food: food.to_owned(),
+            },
             _ => panic!("Unknown command {}", describe_line(&line)),
         });
     }