add build.filter

[eivindfjeldstad]

This allows people to do

build.filter('templates', function (build, conf){
  ...
  if (ext != '.jade') return true;
  file.contents = jade(file.contents);
  conf.scripts.push(file);
  return false;
});

see #157

[bmcmahen]

+1 Not sure how to handle jade atm without something like this.