[cli] Expose configuration API to allow embedding in other Golang projects

[Xayton]

The package configuration, currently internal, should be exported as public API in some way to allow embedding the CLI into golang projects. The configuration package contains a singleton configuration shared among all the instances created.

A possible solution may be to allow pushing an initial configuration from the Init command and keep a different configuration for each instance. In this case, the Settings* API must be changed to operate on a given instance (currently they act on the global configuration).