Overwriting the From header in rss2email

So here is the patch that allows you to do it.

Basically it is useful when the blog has no comment functionality, as in this case you can just specify an address in config.py, like:

OVERRIDE_FROM = {
                'http://landley.net/rss.xml': '''"Rob Landley's blog thing." <rob@landley.net>'''
                }

Without the patch you can just overwrite the name, not the address as well.