Calling on func values from selectors

[zeebo]

We should be able to call methods or function types from selectors. For example, given

type foo struct {
    bar string
    baz string
}

func (f foo) foo(s string) interface{} {
    return s
}

and we execute a template with the context as a value of type foo, we should be able to do

{% .bar %}
{% call .foo .bar %}

or if we had a context of value

map[string]interface{}{
    "foo": func(s string) interface{} {
        return s
    },
    "bar": "bar",
}

we should be able to do the same template as before.

[zeebo]

Commenting to add the fact that

type foo struct {
    foo func(string) interface{}
}

should work as above. Etc for interfaces. Basically anything that can be called should be able to be called from a selector.