Add a configuration hook to customizing how URLs are generated.

Allow a callback in the configuration that makes it easier to customize
how the simple_url is generated. The simple_url value is used to
identify 'unique' routes or blocks of application logic.

Keep the existing default of trimming out numeric GET parameters, so
people upgrading don't get wildly different URL's than before.
Fix URL view not getting the simplified URL as an argument.

Getting the non-simplified URL makes it really hard to compare somewhat
similar URLs.
Clean up trailing spaces.
Update docs for new config option.
Merge pull request #75 from preinheimer/custom-simple-url

Custom simple url