add management of add-ons

Goal

Define and manage add-on to pypushflow.

/close #2 (closed)

Info

This PR come with two add-on project:

• a project to explain add-on with an hello-world (https://gitlab.esrf.fr/workflow/ppfhelloworld)
• a functional add-on 'mongodb' to publish workflow execution on a mongodb database. (https://gitlab.esrf.fr/workflow/ppfmongodb)