Define a sparqleval plugin for BGP query opt

[mwatts15]

There is a recommendation to call optimize_sparql before doing SPARQL queries which "monkey-patches" the evalBGP method in RDFLib to use a more efficient implementation for HDT. This approach has the disadvantage that it requires an additional step for the user of the module where one should not be needed: There is a setuptools "entry point" defined for RDFLib SPARQL eval called rdf.plugins.sparqleval which should allow for a similar substitution without requiring a user of HDT to add the optimize_sparql call into their program initialization. If rdflib-hdt is meant to work in environments without setuptools/pkg_resources machinery, then the optimize_sparql method can still be modified to add to the CUSTOM_EVALS dictionary rather than modify evalBGP in rdflib.plugins.sparql which has the same effect as defining the entry point.