Add support for disabling features about jinjava

[jaredstehler]

Sounds interesting. Would we simply disable whole tags, like {% for ... %} ... {% endfor %} would resolve to ""?

[btiernay]

I was hoping that they would be passed through, as is. Alternatively it could warn or fail, but I think this would be harder to implement.

The reason for the feature request is that I am building a system very similar to Ansible, and I agree with their philosophy here. Brace expressions for YAML, and full Jinja2 for templates. This would help me build such a system in Java.

[jaredstehler]

I think I follow. If you open a PR I'll be able to visualize it and comment further.

[btiernay]

Will do, thanks!

[boulter]

Fixed via #83