This adds a set of configuration variables to the top of post-review, one for each of the command-line switches where it would make sense, so that sites can easily set site-wide defaults.  

I also made the command-line option definitions more consistent, and added '-n' as a short flag for the '--output-diff' option (since '-n' usually means "tell me what you're going to do without actually doing it," which seems appropriate here and is how I use that option).

Finally, I updated the usage string.

I've done basic sanity-checking on each of the configuration variables.