I've recently had to subclass optparse.OptionParser, and copy-paste the
exit method, just to change where errors were printed to (I needed
stdout rather than stderr).  I've also had a request from a client to
log errors with command-line parsing to a file, rather than to stdout.

So, this patch adds an error_log parameter to OptionParser.__init__
which is used instead of stderr internally (and, of course, defaults to
stderr).