Make sure a link with a year range encompasses the whole range.
Make sure a link with a year range encompasses the whole range.

Previously you'd get [LINK 1996]--[LINK 2003], where both
links went to the same citation.  Now you get [LINK 1996-2003].

Closes #146.

adunning/pandoc-citeproc,jgm/pandoc-citeproc,jkr/pandoc-citeproc