Allow to add username to OAuth config

[nichtich]

A configuration file with both username and oauth leads to an error:

Error: credentials can not be both oauth tokens, and a username and password

But a username is helpful to document which user OAuth credentials belong to. The error should only be thrown if all of the fields username, password, and oauth are given.

[maxlath]

the username and password fields are constrained, but you could use any other value, for example you could set _username to document the corresponding user

[nichtich]

Thanks, I'll use description!