%(prog)s format specifier is not converted in description and epilog

[schlepuetz]

The %(prog)s format specifier of argparse does not seem to be picked up and replaced by the program name in the description and epilog arguments to argparse.

I frequently add usage examples in the epilog of the following form:

epilog = ('''
Examples:

* Run the script with the verbose option:

    %(prog)s -v
''')

This issue seems to have been partially addressed in Fix #27, and the solution works well for the help arguments of individual argument definitions, as well as the parser`s usage argument. However, it does not seem to be applied to the ```description``` and ```epilog```.

So it seems the solution already exists and would hopefully not be too difficult to extend to include description and epilog as well.

Thanks for a great Sphinx extension, by the way!

[schlepuetz]

Sorry, I just saw right now that exactly this issue seems to be already addressed in a pull requets:
#113