Plugins to add their CLI

[azukaar]

For now, plugins can only hook into existing CLIs.
Allow plugins to define their own CLIs.

- provider-react
    - add-component.gs

g -p react add-component

Need re-design on the architecture, since this is not ideal in term of allowing users to use custom CLI without explicitly calling the plugins. May be g add-component or g react-add-component should look into the plugins folder without the --provider option