Add option to use `graph` as keyword instead of `graphs`

[x-m-el]

For the yarrrml spec, it might be okay to use graph instead of graphs as a keyword, more info about this discussion at citiususc/yatter#78

E.g. the following should be correct, following the spec (currently the spec only mentions graphs)

mappings:
  test:
    graph: https://example.com/$(id)
    s: https://example.com/$(id)
    po:
      - [a, ext:Test]