Enhance citation-finder to deal with partial U.S. citations lacking page numbers

[brianwc]

The Supreme Court often cites its recent cases that don't yet have a full U.S. citation like so:

Bullcoming v. New Mexico, 564 U. S. ___ (2011).

We have Bullcoming v. New Mexico, know it is a SCOTUS case, and know it was filed in 2011. Sometimes, but not always, that would be enough information to uniquely identify the document we have as the one the court is citing. So, we should implement this for the times where it would work.

[mlissner]

freelawproject/eyecite#30 is now tracking the parsing part of this. We can use this issue to track the citation matching part.