Add support for +=, -=, ~= ...

It would be really nice if we could increment, decrement and append strings just by saying

```
{% set dummy=1 %}
{% set dummy+=1 %}

```
or

```
{% set dummy="foo" %}
{% set dummy~="bar %}
```

Otherwise we always need to write `dummy=dummy+=1` which is repetitive. I think the syntax above would be very idiomatic because it exists in PHP and comes naturally.
