Add support for :noindex:

Sphinx 2.1 became more strict and now complains about duplicated object descriptions:

```

Warning, treated as error:
duplicate object description of astropy.nddata.utils, other instance in nddata/index, use :noindex: for one of them
Sphinx Documentation subprocess failed with return code 2
Exited with code 2
```

However currently the `automodapi` directive doesn't support `noindex` (`WARNING: Found additional options noindex in automodapi.`)
and `no-heading` doesn't do the trick.

Start by locating the automodapi directive and its option parsing, then reproduce the reported Sphinx documentation build with the noindex option. Done means automodapi accepts :noindex: without the additional-options warning and the duplicated object description no longer causes the documentation build to fail.

Written by the indexing model from the issue text.