Add support multiple notebooks in a single nblink file

[aaraney]

This PR adds the ability to include multiple notebooks in a single .nblink file. The semantics to achieve this are as follows:

   [
        {
            "path": "relative/path/to/notebook"
        },
        {
            "path": "relative/path/to/notebook_2"
        }
    ]

So, in short, instead of supplying a single JSON object, instead a list of JSON objects are provided in the .nblink file.

closes #12

[aaraney]

Just wanted to note here, for now ive created a new forked project called nbsphinx-multilink that adds the functionality provided in this PR. @vidartf you are still the "author" of the package, I have listed myself in the maintainer role and will publish the forked version on pypi for now. @vidartf is fully credited with the packages authorship.