Support for multiple parallel footnote sets

[memartin]

I'd find it grand if one could specify something like a realm for footnotes.

Something like this:

Testing out the !FootNoteMacro[[FootNote(foo,Here is the first footnote.)]]. 
This will be a good test to exercise all of its functionality
[[FootNote(foo,Here is the second footnote)]]. And now I'll add a 
footnote[[FootNote(bar,http://trac-hacks.org/wiki/FootNoteMacro)]] 
of a second set, like for a collection of external link 
references[[FootNote(bar,http://en.wikipedia.org/wiki/Reference)]].

== Links ==

[[FootNote(bar)]]

== Addendum ==

[[FootNote(foo)]]

It would, for example, allow collecting different sets of Footnotes, using them arbtitrarily throughout the document, and not only one new set after another set has been displayed.

Different formattings could be achieved through custom CSS (or style parameter), like having "normal" sets formatted like they are at the moment, whereas other sets (e.g., external References) could be displayed [1] like this [2].

[1] ​http://en.wikipedia.org/wiki/Display
[2] ​http://en.wiktionary.org/wiki/this

What do you think?