Configuration via external addon.json

[yakky]

addon.json is not really required to be in the application package

We can easily add support for a json file provided with an option that contains:

• a list of addons configuration
• package name can be used as a requirement passed to pip for installation

sample usage:

djangocms mysite
cd mysite
python -mapp_enabler install --addons=/path/to/addons.json
python manage.py migrate