Adjust bibliography paths when gathering

[jdutant]

When gathering bibliography fields from sources, adjust relative paths. On by default, option to deactivate.

[jdutant]

Not sure this is needed. At the moment we add the source file's folder to the resource-path. This covers the case in which source file gives resources (incl. bib) in paths relative to itself. If it gives sources in relative paths, but not relative to itself, then there is no way we can figure out the difference in relative path from master.md and from source.md.

We may want to add an option do-not-add-resource-path in case users don't want pandoc to search files relative to the source's folder.