code: function normalize (val, safe) {
      return (
        val == null ? ''
        : val === false ? ''
        : Array.isArray(val) ? val.map(normalize).join('')
        : safe === false ? $.escapeHTML(val)
        : String(val)
      )
    }
docs: Function that normalizes interpolation substitions.
  It flattens arrays while ignoring falsey values.
  All other values are converted to strings.