Add support for OAuth token usage

[linkmauve]

This package currently requires a password for login, it should instead allow for OAuth token to be provided.

See also https://docs.mattermost.com/developer/personal-access-tokens.html

[Vaelor]

Yes, that has to be done. I already had a look at it and it seems to be easily integrated, I do plan to include this in the next release.

[Vaelor]

@linkmauve Please have a look at the new 2.2.0 version if it fixes this issue for you.

[Vaelor]

Assuming it works like expected, at least in my case. Please reopen if not ;-)