Configureable needs.json builder

[danwos]

To allow users to define the content of a needs.json file and to keep multiple of these configurations available without making changes in the conf.py file, dynamic needs.json builders shall be registerable.

Example conf.py:

needs_builders = {
  "my_needs": {
    "contains": [...]
    "excludes": [...]
    "filename": "my_own_needs.json"
  }
  "my_builder_2":  {...}
}

The different builders shall be available by their key.
Example: sphinx-build -b my_needs . _build/ or make my_needs.

contains defines a list of values, which shall be part of the file. All not-mentioned values are not taken into account.

excludes takes everything into account, also all internal values. But removes the defined values.

filename is the name of the file under which the data gets stored. needs.json is not allowed, as a needs.json shall follow a certain standard.