allow ending through truncate, just like nunjucks would let you do

Originally Nunjucks provides the following function
    truncate: function(input, length, killwords, end)
Apos also has the possibilty to add an ending 
    self.truncatePlaintext = function(str, length, pruneStr)

Then Nunjucks truncate function is overwritten by apos, but originally without the ending
    nunjucksEnv.addFilter('truncate', function(data, limit) {
      return self.truncatePlaintext(data, limit);
    });

this small fix lets it through so I can change the ending dynamically ;)