Add subdirectory support for reverse proxy

[Tom60chat]

I added support for subdirectory, which allows the application to be accessible like this: https://myserver.com/mineos/.
Just change overwrite_web_root in mineos.conf.

Exemple:

use_https = true
socket_host = '0.0.0.0'
socket_port = 8443
base_directory = '/var/games/minecraft'
overwrite_web_root = '/mineos'

...

MineOS will now be available at https://myserveur.com/mineos/

See: #442