Variants for need options

[iSOLveIT]

This PR implements variant handling based on need options.

Usecase 1
In conf.py:

needs_variants = {
  "var_a": "'var_a' in sphinx_tags"  # filter_string
  "var_b": "assignee == 'me'"
}

In any .rst file:

.. req:: Example
   :id: x
   :status: var_a:open, var_b:closed, unknown

From the above example, there are pre-defined variants, which have a “filter string” that defines to which variant a need belongs in the current situations.

Usecase 2 - Filter in option
There shall also be a way to use the filter string inside the options

.. req:: Example
   :id: x
   :status: ['var_a' in tags]:open, [assignee == 'me']:closed, unknown 

Default option
The last option can exist without a filter, so we consider it if no other options fulfil their filter rules.

[danwos]

Only small findings in the docs.

We should also add already the Sphinx-Tags support.
Then the feature is already complete and users can go on bug hunt.

[danwos]

Thanks for the great work.