Add an autoinject option to provider.

[mwiatrzyk]

I had following provider added to a project I work on:

@provider.provides('listener')
def start_listening():
    def listener(event):
        ...
    unsubscribe = subscribe(listener)
    try:
        yield
    finally:
        unsubscribe()

It is responsible for collecting events when application is running, and no direct use was expected for that provider. Currently, to start such listener, the provider needs to be directly injected to the code:

injector.inject('listener')

But this is not very handy, as the provider is not expected to return any object, and is error-prone when called too late.

The solution will be to add autoinject=True|False (with False as default) to the provides decorator, so the injector will automatically create such providers when scope is entered, and destroy when scope is left:

@provider.provides('listener', autoinject=True)
def start_listening():
    def listener(event):
        ...
    unsubscribe = subscribe(listener)
    try:
        yield
    finally:
        unsubscribe()