Provide an interface for plugin to declare derex related operations

[chiruzzimarco]

We need an interface for plugins to declare function needed to handle some operations, namely:

• additional steps needed to provision the plugin (e.g. create a database or an elasticsearch index). I'm thinking about generic setup method which, if exists, will be called by derex when provisioning the plugin
• a reset method which will be called by derex when resetting the project after New command reset-project #56 is implemented
• a backup and a restore method which will be called by derex when backupping/restoring the project

Those are just some examples, the generic purpose for this is to provide plugins with hooks in project wide operations.