Add .buffrs/config.toml

[dgehriger]

Based on upstream version 0.8.1 but with following additions:

• Introduction of a configuration file in .buffrs/config.toml (in the current working directory, or any parent)
• Config file allows specifying:
  • List of registry URLs and their names
  • A default registry

Example:

[registries]
some_org = "https://artifactory.example.com/artifactory/some-org"

[registry]
default = "some_org"