Add from_input in Rdf_xml.

Xmlm allows one to set some options for reading XML input streams as
optional arguments of Xmlm.make_input.

This patch adds a Rdf_xml.from_input function which populates a RDF
graph from a Xmlm.input handle, allowing one to completely customize how
Xmlm reads XML data.

This is in particular useful when parsing XML files which declare named
entities in their DTD: Xmlm does not parse these declarations, but we
can workaround this limitation by using the ?entity argument of
Xmlm.make_input.