Support login with username/password via Plex

[SpaceK33z]

Finding the Plex token is not easy for non-tech-savvy people. It would be a lot easier if you could just enter your username and password from Plex, and the extension would generate a token.

The username/password should not be stored, only the resulting token.

Also see plex docs about this: https://forums.plex.tv/discussion/129922/how-to-request-a-x-plex-token-token-for-your-app/p1