Fix bug with relative addresses

hidden index files without trailing slashes would get overridden when clicking a
relative link. artemis now adds a trailing slash to every directory index.

Fixes: https://todo.sr.ht/~sfr/artemis/2
Add support for non-gemini files

all non-gmi files will be sent with a text/plain header

Implements: https://todo.sr.ht/~sfr/artemis/1
Add support for custom userdir path, set defaults