Feature request: load sensitive environment variables in yaml, or directly

[BentEngbers]

Thanks for the awesome bot!

Is your feature request related to a problem? Please describe.

I would like to commit my config.yaml file used for go-neb to my repository. I cannot do this at the moment, because it will leak my matrix accessToken, DeviceID and my github access token.

Describe the solution you'd like
I would like to supply these critical pieces of information through the environment. Either to use a ${} or $() notation in the yaml file or directly using manditory environment variables.

Describe alternatives you've considered
I have considered using the api of go-neb bot without a configuration. there are two problems with that approach.

1. I do not expect to change often, so a single config file is a lot easier/simpeler.
2. I use the github integration feature. this requires me to have a public facing address for webhooks.
   Please correct me if I am wrong but the public facing port for the webhooks is the same as the admin port. I do not know how i would secure such a setup.