def set_extra(self, extra):
self.extra = extra<EOL>
specify a custom info appended to the end of the next message

TODO:
    - [ ] extra is a bad name; come up with something better and rename

Example:
    >>> import progiter
    >>> prog = progiter.ProgIter(range(100, 300, 100), show_times=False, verbose=3)
    >>> for n in prog:
    >>>     prog.set_extra('processesing num {}'.format(n))
    0/2...
    1/2...processesing num 100
    2/2...processesing num 200