Add Jekyll site url option

[chrismytton]

The Jekyll templates that are being rendered will often contain links that will only work on the Jekyll version of the site, not the Sinatra version. The Jekyll site needs to prefix all of these with {{ site.url }} so that they are absolute links including a hostname rather than relative.

It would be nice to add a way to dynamically set the site.url from Sinatra so that the links go to the correct place.

As I type this I wonder if there should be a more general mechanism for interacting with the site's configuration, so that other values can be adjusted as needed?