Allow overwritten on render_template to use Flask-Themes based app mattupstate/flask-security#179

I am sending this PR just for you to see if it is a good way, or it there is a better idea..

remembering the goal is allow this:

```python
from flask.ext.security import Security as _Security
from myapp.templates import custom_render_template

class Security(_Security):
    def render_template(self, *args, **kwargs):
        return custom_render_template(*args, **kwargs)
```

https://github.com/mattupstate/flask-security/issues/179