Improve support for user defined image directories

[banool]

Currently there is decent support for text files containing links. The support for directories however is pretty rudimentary. You can see this in the getImagePaths function in server.go.

This would probably require research into each popular webserver's directory listing, as well as creating a sane default. As such it'd probably be a high effort low impact change. A way to help with this might be only pulling links from anchor tags that have image extensions, though I've been reluctant to specify a whitelist of image extensions since browsers support so many different types of media files (though it would only have to be what is supported by the img tag). It's also a bad idea if we move into a world with video support (see #17).