`manage.py` should be easier to extend

[Zygmunt Krynicki]

The manage.py command should be easier to extend and preferably using a standard and supported API.

During weekend hacking on a Go provider it was discovered that the standard set of manage.py subcommands was insufficient to build executables from source. That particular bug is being fixed now but it is not inconceivable that similar limitations will be found again later. To allow the developer community to come up with standardized best practices for things not already covered by plainbox, we should encourage and support such behaviour right in the core (that is, extensibility).

I would like to propose that we offer the @manage_py_extension class decorator as a part of standard provider_manage.py API.