Support trailing parens in crossref title

[analog-cbarber]

Sometimes people like to include parens when referring to a function or method call in a doc string, but
to do so using this package requires you to repeat the target name, e.g.:

def bar():
    """
    This function invokes [foo()][..foo]
    """

it would be nicer if we automatically ignored the trailing parens so that you could write:

def bar():
    """
    This function invokes [foo()][..]
    """

or even

def bar():
    """
    This function invokes [foo(x=42)][..]
    """