Support for @blueprint.cli.command

It would be great to have access to the functionality of `app.cli.command` via blueprints - i.e. something like:

``` python
blueprint = Blueprint('myblueprint', __name__,)

@blueprint.cli.command()
def doblueprintstuff():
    """Initialize the database."""
    #....
```

It would help a lot when building modular applications.
