Links between citations and bibliography entries were broken in XHTML

Links between citations and bibliography entries were broken in XHTML
output, due to failure to clean the ids in the new citation stuff.

I've solved this by allowing the citation format information to contain
keys of the form "clean:key". This signals that we are to apply the
html::cleanAttr() function to the key before returning it. I.e., we
strip non-alphanumeric stuff, basically.