Add additional Jinja2 template overrides

Add the following Jinja2 template overrides:

* `variable_start_string`
* `variable_end_string`
* `block_start_string`
* `block_end_string`

Useful for when templates need to render templates that use similar syntax. More specifically, I'm rendering a `consul-template` template with Ansible which also uses double curly braces for interpolation, but I need Ansible variables rendered in the output as well.