Add affiliations to documents

[timtroendle]

It would be good to be able to show affiliations. However, this isn't easily possible across different output types.

One would need to change pandoc's html and docx template and have a redundant structure in the metadata. The latter is because one would want to keep the original author fields as they are used for metadata and still have a structure linking authors to affiliations.

For these reasons, a filter seems like the most useful.

[timtroendle]

I've tried the existing lua filters author-info-blocks and scholarly-metadata but they failed.