Enable ELLIPSIS by default for doctests

And made it possible to set/override doctest flags when calling
`tangled.util.load_doctests()`.
Allow module name passed to load_object() to be empty

Before, you could do this:

    load_object('.:some_attribute')

Now, you can also do this:

    load_object(':some_attribute')

Also added some doctests for load_object().