Merge changes from plain.bst version 0.99b into assignment.bst

The file assignment.bst was based on plain.bst from BibTex 0.99a. Its
license does not permit redistribution. This was fixed in version
0.99b (found in texlive-base 2017.20171031).

This commit is a rebase of the annote and documentURL features on top
of the new version.