Add support for config file in image directory

This config file will be loaded together with the images and defines the
passwords for login and some other parameters.

config.yaml.example

@@ -0,0 +1,14 @@
+# Save this file as config.yaml into the image directory
+
+# Config and image list will be updated every 60s. If there is
+# an error with the config (or no config at all), no access to
+# the images will be granted; check the log in this case of
+# errors. Changing the config invalidates all active sessions.
+
+title: Website title
+passwords:
+- password1
+- password2
+
+# Optional, will be displayed above images
+top-message: "Please do not share or save these images"