Split features using hooks plugins

[claustres]

Krawler already allows to plug new hooks dynamically by code. Thus, it is easy to create plugins to make it more modular and remove some (most ?) hooks from the core.

However, in order to make it continue working without any specific code (i.e. simple job file) we need to 'load' required plugins for the job on-the-fly and register hooks.

For instance node-gdal is a big dependency that would be better to integrate through a plugin.