Convenience wrappers for show_backref_chain(obj, inspect.ismodule)

[mgedmin]

Using objgraph.show_chain(objgraph.find_backref_chain(...)) is a lot of typing. Provide convenience wrappers.

[mgedmin]

One of the goals of #5 is to make queries like this fall out naturally from the new-and-improved API design. Hmm.

>>> g = objgraph.why_alive(obj)
>>> g.show()