Allow functions to be used for preprocessors

Allow functions to be used instead of strings. Better for extending testacular with new preprocessors:

    var pogo = require('pogo');

    preprocessors = {
      '**/*.coffee': 'coffee',
      '**/*.pogo': function (content, filename, done) {
        done({content: pogo.compile(content, {filename: filename})});
      }
    };

Let me know if there's a better way to do this... Cheers.

Tim.