Allow providing a link mapping for missing pages

When importing Confluence space exports, links to pages outside the space will break because information about these pages is needed but missing.

This can be worked around by providing a link mapping, that maps space key document titles to XWiki entity reference. This mapping can be gathered through previous imports.

We shall:

• add an input filter stream property to provide such a link mapping to Confluence-XML
• add a method that will allow users to get a link mapping built from the Confluence package.