Decouple commands registration and invocation context from `urfave/cli`

Description

This merge requests decouples commands registration / command invocation context from urfave/cli package.

This package is meant to be used only in commands module, and everywhere else we abstract it away.

We also plan to introduce a new context for Execute() that is going to be an internal type that is extensible. This way we can add more features to the context, like Go-specific Context and abstraction for environment variables that is much more testable.

Closes #3 (closed)

/cc @tmaczukin