Introduce generic syntax for macros

The old MediaWiki syntax parser had a custom syntax for inserting rendering macros. There is not exact equivalent in MediaWiki syntax (some things are translated as macros, some don't) so we need to find something which allows explicitly asking for a macro.

Since the extension mecanism syntax is based on XML elements in MediaWiki world and the actual parsing done by bliki I propose the following solution which does not require to patch bliki:

<macro:mymacro param1="value1" param2="value2">
some black box content
</macro>

i.e. any element prefixes with "macro" which end up as macro in the rendering events.