Convert links to any material

[jdutant]

A document can hyperlink to files of any type. When links have relative targets, we need to make sure that they're correct in the resulting document. What are the options?

1. Mimick pandoc's behaviour. Let the source files relative links as they are.
2. Adjust to make them relative to the master file directory.
3. Adjust to make them relative to the present working directory.
4. Adjust to make them relative to the output file directory.

3 is needed to make image inclusion in PDF/epub and extract-media work. We should cater to 1 in case the user is already entering the links with a specific root in mind. We might want to cater for 2 as it mimicks having writen a big master document directly with links relative to it. We might want to cater for 4 to ensure outputs work out

I suggest implementing 3 and 1. Users can achieve 2 and 4 by running pandoc from the master or output directory, respectively and relying on 3.