Add support for lambda 'from' matchers.

I'm working on a CMS type app where the user needs to be able to change urls on the fly. I want to redirect old urls to the new urls appropriately, but Rack::Rewrite doesn't allow dynamic matchers. With this change, Rack::Rewrite can pick up the changes from my app immediately rather than on the next restart. 

I'm sure there are plenty of other uses for this as well.

Sorry about the commit noise -- my editor auto-drops trailing whitespace.