Add display abstraction

In case we want to run this on something that isn't an ANSI terminal, we
have the option to implement it however we want.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

agame/display/display.py

@@ -0,0 +1,32 @@
+import abc
+from typing import Optional
+
+
+class Display(metaclass=abc.ABCMeta):
+    """
+    Displays are an abstraction over printing things to the screen.
+
+    Use a specific implementation based on the kind of display you want to use.
+    """
+
+    @abc.abstractmethod
+    def line(self, line: str = ""):
+        "Print a single line to the display."
+
+    @abc.abstractmethod
+    def input(self) -> str:
+        "Get player input."
+
+    @abc.abstractmethod
+    def wait_for_ack(self, prompt: Optional[str] = None):
+        """
+        Wait for the user to press a key.
+
+        The behavior is up to the implementor. This could be something like
+        "press any key to continue", "press enter to continue", or "click OK to
+        continue". This method should block until input is acknowleged by the
+        user.
+        """
+
+    def finish(self):
+        "Final cleanup code for this display."