add an image_file driver for development

The driver just writes images to files. This makes it possible to test without
an e-ink display.

inkycal/display/drivers/image_file.py

@@ -0,0 +1,17 @@
+# Display resolution
+EPD_WIDTH       = 800
+EPD_HEIGHT      = 480
+
+class EPD:
+    def init(self):
+        pass
+
+    def display(self, image):
+        image.save('display_image.png')
+
+    def getbuffer(self, image):
+        image.save('getbuffer_image.png')
+        return image
+
+    def sleep(self):
+        pass