User macros should be able to call macro plugins and user macros

When writing a user macro, it should be possible to call an exisiting macro. Since the bulk of confluence wiki rendering is encapsulated in macros, not being able to call existing macros greatly reduces the value of user macros. Actually the same issue really exists for macro plugins, since it is not particularly easy to reuse an existing macro plugin from a macro plugin.

Our use case is to have special macros for different jiraissues, like:

which just turns around and calls

with an appropriately crafted url.