Add an interdoc xref resolver callback (was: Add option to control interdoc xref prefix)

[mojavelinux]

Add an option named :relfileprefix to control the prefix that is prepended to interdocument references (i.e., source to source links). This option is important for Antora, which requires interdocument references to be relative to the root of the catalog rather than relative to the current file.

[mojavelinux]

Since xrefs are often very system dependent, which might be even better is to add a callback to resolve the target. That would depend on #31.