Extend graph visualization to give more flexibility in filtering

[ubyndr]

Resolves #38

I have refactored the subgraph generation in visualize_rdf_graph method. Couple of example queries and results can be seen below. I'm open to parameter re-naming.

gg.visualize_rdf_graph(
    node_selector={"property":"label", "value":"epithelial cell of glomerular capsule"},
    predicate=None,
    file_path="kidney_iri_test.ttl",
    bottom_up=True
)

gg.visualize_rdf_graph(
    node_selector={"property":"label", "value":"epithelial cell of glomerular capsule"},
    predicate=None,
    file_path="kidney_iri_test.ttl",
    bottom_up=False
)

gg.visualize_rdf_graph(
    # start_node=["http://example.org/0088cc46-ecea-42bc-83d4-e0d51e9cffcf"],
    node_selector={"property":"subclass.l1", "value":"PEC"},
    file_path="kidney_iri_test.ttl",
)