[Versa] Named graph/context syntax

[Chimezie Ogbuji]

This is more of a FYI, regarding syntax for associating an explicit
name with a context (or subgraph).  There is some precedent that is
eerily similar to the syntax we are developing:

http://www.wiwiss.fu-berlin.de/suhl/bizer/TriG

The syntax names graph with the following two conventions:

:graphName { .. statements ..}
:graphName :- { statements ..}

Chimezie