Extension point for custom extensions

* New base.package property added, to define the base package where to
scan for custom extensions
* New dependency added (Reflections) to be able to scan classes
* ReflectHelper updated with 2 new methods allowing to scan for custom
features and plugins
* New features / dt:features table attributes, allowing to enable one or
more custom features in a table
* New plugins / dt:plugins table attributes, allowing to enable one or
more custom plugins in a table
* WebResourceGenerator refactored by externalizing code in new managers
: ExtensionManager and ExportManager
* Linked with dandelion/issues#108