Added a new hook to add new element rules to the Whitelister.

Created a new hook 'construct_whitelister_element_rules' to add new element rules to the Whitelister class. This follows the suggestions in https://github.com/torchbox/wagtail/pull/309.

Example:
```{.python}
def whitelister_element_rules():
    return {
        'blockquote': allow_without_attributes,
    }

hooks.register('construct_whitelister_element_rules',
               whitelister_element_rules)
```