define(name, fn, raw) {
    fn = utils.coerce(fn, raw);

    if (fn.nodeName) {
      this.options.globals[name] = fn;
      return this;
    }

    // function
    this.options.functions[name] = fn;
    if (undefined != raw) fn.raw = raw;
    return this;
  }
Define function or global var with the given `name`. Optionally
the function may accept full expressions, by setting `raw`
to `true`.

@param {String} name
@param {Function|Node} fn
@param {Boolean} [raw]
@return {Renderer} for chaining
@api public