Add home screen (#42)

## Summary

* Add home screen
* Sits as new root screen, allows for navigation to settings or file
list

src/screens/EpubReaderScreen.cpp

@@ -98,7 +98,7 @@ void EpubReaderScreen::handleInput() {
   }
 
   if (inputManager.wasPressed(InputManager::BTN_BACK)) {
-    onGoHome();
+    onGoBack();
     return;
   }