Support keyring in git-obs

[dmach]

It might be nice to support reading tokens from a keyring rather than storing them in a plain-text file.

[johanneskastl]

Maybe allowing to specify a command to retrieve the key would also be an option, this way a user can get the token from whichever password safe etc. she is using.

• gopass
• gpg decrypt ...
• keepassxc-cli
• ...