code: function missingGlyphs(glyphs, svgDir, cb) {
    async.reject(glyphs, function(glyph, cb) {
      var filenames = glyph.filenames().map(function(filename) {
        return svgDir + '/' + filename;
      });
      async.every(filenames, fs.exists, cb);
    }, cb);
  }
docs: Filters all glyphs to returns only the ones missing on the FS