Add generic parameters support to Figure and Link events

For example XWiki has the following syntax for image macros and links:

• image: http://code.xwiki.org/xwiki/bin/view/Macros/ImageMacro
• links: http://platform.xwiki.org/xwiki/bin/view/Main/XWikiSyntax#HLinks

For the image macro there are the "document" and "fromincludingdoc" which are specific to XWiki and thus cannot be put as standard parameters.
Same for links.

Thus I propose to allow parsers to pass a Map of properties (pair/values) to the Sink API so that sinks can be written to understand them (the XWiki sink would understand them for example).