don't hyperlink historical DC Code citations

e.g. "2001 Ed., § 29-241.03." in Prior Codifications sections

fixes #11
pass a 'dc_code' context to the dc_law citation processor (see https://github.com/unitedstates/citation/commit/717fbbe47dd42291d6775d9dd640de415275d8ec)
make a Title-Section ID for placeholder sections when the placeholder is for a *single* section too so that we can automatically link to them