Take 'form' for date-part elements from date if not specified.
Take 'form' for date-part elements from date if not specified.

Previously if the 'form' were unspecified in a date-part
element, it would go to the default 'long', even if the
date as a whole was 'numeric'.

Closes #116.

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