Python parity bug(?): Comma-delimited expression

The following template compiles with Python jinja2 (and yields an output of "1"), but does not work with jinja2cpp.
```
{% if 0, 'foo' != 'bar' %}
1
{% endif %}
```

Here's the jinja2cpp error that I get.
```
LOAD ERROR: noname.j2tpl:2:8: error: Expected end of statement, got: ','
{% if 0, 'foo' != 'bar' %}
---^-------
```

I'm actually not entirely sure what this expression is supposed to mean, or if it is just incidentally supported due to the Python jinja2 implementation.

I was not able to find an explicit call out for what this would mean in the documentation, and I wasn't able to discern how this might be useful from mucking around experimentally with a variety of values.
https://jinja.palletsprojects.com/en/2.10.x/templates/

So unclear if it would make sense to fix in jinja2cpp for parity, or if Python jinja2 should be fixed to not support this "bad template syntax".