Allow separate curl_opts for before and after

[SomeBdyElse]

Right now it seems impossible to define independent curl_opts (such as basic auth credentials) for before and after. Both sections will be overridden by the global curl_opts, even if the global curl_opts are empty.

https://github.com/evolvingweb/sitediff/blob/master/lib/sitediff/config.rb#L111