Added references field to options in markdown reader.
Added references field to options in markdown reader.

This allows the user to pass in a list of references to
be used in resolving links.

jgm/lunamark,tst2005/lunamark,simoncozens/lunamark,tst2005/lunamark,simoncozens/lunamark,simoncozens/lunamark,tst2005/lunamark,jgm/lunamark,jgm/lunamark