add context function

It's useful to create helper function that can be used in the template. for example:

```php
function _p($v, $d = null) {
  extract(context());
  return isset($$v) ? $$v : $d;
}
```

Then you can use function `_p` in your template:

```php
<?= _p('site_title', 'no title'); ?>
```