Add syntax for argument type declaration for macros

Supporting this syntax in addition to #513 might be nice to have:

```jinja
{#                    v                      #}
{% macro test(title: Option<&'static str> = None) %}
    {% if let Some(title) = title %}
        Title: {{title}}
    {% endif %}
{% endmacro %}

{% call test() %}{% endcall %}
```