Support for formatting

Hi,

Jinja has this nice support for formatting like:

```HTML+Django
{{ '0x%0X' % payload.id }}
```
It would be nice to have something like this in tera, specially for integers file types. Maybe something more rust like:
```HTML+Django
0x{{ payload.id | format(":X") }}
or 
0x{{ payload.id | format=":X" }}
```