Provide easy way to reference figures in other documents

[humburg]

Currently the only way to refer to a figure in a different document, whether it is a child, sibling or parent, is to explicitly link to the corresponding anchor in that document. It would be nice to have a shorthand for this, especially if it could make use of the automatic figure numbering.

[humburg]

This is at least partially addressed by #6. With these changes it is now possible to link to Figures and Tables in child documents using the same function used for intra-document references (i.e. figRef and tabRef respectively).

This still doesn't work for links to documents that aren't dependencies of the current document. The reason for this is that the current implementation relies on an index of figures and tables that is generated for each file as it is being processed. This requires the link target to be processed before the referencing document and that is only guaranteed for dependencies.