Feature enhanced config

[FalkNisius]

• you have in the pipeline an old pull request for managing tokens external with or without a refresh token, and use only the access_token for authentication
• on my use case with serveral office365.com accounts as relay, I have to handle oauth clients with client credentials flow, and I was not able to use the device flow for some tenants
• I have add some config vars for this case (config.h / config.cc)
• I allow the same vars for override (token_store.h / token_store.cc)
• add the description in the sasl-xoauth2.conf mark down
• and add a very short description in the read me

TODO

• Open is in from my side the enhancement for the sasl-xoauth2-tool, my proposal is to allow calling with a config json, to prevent blooting out all possible options to the command line