Support token-based authentication alongside password-based auth

[alexgleason]

The access token should be retrieved beforehand rather than saving the password for the bot.

[xurizaemon]

I support this provided we preserve support for the existing auth method also.

Access tokens in Matrix may be configured to have a limited lifetime (eg 7 days), so there's a use case for bots being authenticated via password at startup, and using the generated access token to remain connected for the length of that session. Users can then refresh the token by restarting the bot, which regenerates a new token using the password. Happy to consider a PR that adds token-based auth while preserving password-based auth.