support for authentication

[marco-calautti]

The stremio add-ons can request a key to the user in order to install them only when the right key is provided. See for example: https://github.com/Stremio/stremio-addon-with-key

Currently Knightcrawler does not support this, which means that exposing the addon outside the internal network will make it accessible and installable by anybody. This, in my opinion, defeats the main purpose of using Knightcrawler: self hosting it.

It would be very useful if Knightcrawler supports such a feature.