Keying function

[mikeboers]

It could be nice to be able to provide a function that returns the specific key to use much like etagger does:

def keyer(*args, **kwargs):
    id_ = kwargs.get('id')
    if id_ is not None:
        return id_
    else:
        raise NotImplementedError()

@memo(keyer=keyer)
def render_blog_post(content, **kwargs):
    # Some expensive process to render latex, Markdown, etc.
    pass

# ...

render_blog_post(post.body, id=post.id)