Extending the "sort" filter so that it can accept a path as a key (in dot notation)
Added the option for markdown files to optionally be preprocessed by swig.

Not sure if:

 1. this should be a custom middleware rather than part of Haggerston and
 2. if this is part of Haggerston should opt in work like this or via a `markdownFilesToPreprocessWithSwig` array in the Haggerston options (rather than a property on the page)
The sort filter should work correctly when it is passed Date objects.
Outputting some basic information about how long the haggerston task took to run...
Added some additional benchmarking to Haggerston so you can easily see what makes it take time...
A bit faster to reuse the RegExp
And substr seems to be a little faster still...