Adds --double-indent

PEP 8 states that hanging continuation lines should be indented once,
except for blocks:

    somefunc(
        "foo",
        "bar")

    def somefunc(
            param1,
            param2):
	pass

    if (someconditionhere and
            someothercondition):
        pass

However, it is pretty common (ALTHOUGH NON PEP8 COMPLIANT) to indent
twice everywhere instead, eg:

    somefunc(
            "foo",
            "bar")

Without --double-indent, this would trigger E126.