Add rewrite rules, aka the whole point of this thing.

Allow the implementing app to modify the config hash however they want
via Ruby. I didn't go in intending to support multiple apps or config
blocks, but it would. I'm not sure that's a real use case, but whatever.

One tricksy thing here is changing the Nokogiri document in place and
using its #to_html method to generate the new markup. I'm not sure if
this is slow or has any weird implications. I've already seen that it
will write a 'Content-Type' meta tag to the generated markup if you
don't have one in place already, so that's not perfect. I'm going to
roll with it and hope for the best.