Add 'multibib-renumber.lua' filter

[wlupton]

The initial version is just code that has been removed from PR #8.

[wlupton]

I believe that I've fixed at least some of the problems reported by @jerome-white, in that multiple citations per cite should now work (not sure whether that will fix the range problem... probably not), and links should work.

This is quite messy, because the only way that I see to fix the citation numbers is to do string processing on the cite text (this text covers all the cite's citations). I have currently assumed that the citation numbers in the bibliography are in square brackets.

This is largely untested but at least it works for the test input (which I have modified). Obviously the tests need expanding to test across a variety of conditions.

Feedback welcome!

[wlupton]

Re this:

The citation may list "4-7", for example, when in fact it should be "31-34".

I don't think this will have been fixed, because this is presumably part of the cite text (in this case the cite will have four citations and they'll be consecutively numbered and therefore run together into a range). So the parse would have to be more elaborate. But is this really the right thing to do? I have my doubts.