DataTable config per needtable

[danwos]

.. needtable:: shall get an option to define the js-based configuration of the DataTable package.

As this configuration can be quite huge and complex, there shall be a new option needs_datatable_config for the conf.py file:

needs_datatable_config = {
  "config_1": {...},
  "config_2": {...},
}

.. needtable::
   :config: config_1

The config_1 dict can contain any kind of elements. It is set in the JS code as js-dict for the config. So the elements must match the allowed config from DataTables