handle sections for composites

[steve-chavez]

Should be like:

create type person (name text, lastname text);

create or replace function hello(p person) returns text as $$
Hello, {{#p}} {{name}} {{/p}}
$$ language plmustache;

select hello('(John,Doe)');
Hello John

select hello(null);
Hello

According to https://github.com/mustache/spec/blob/master/specs/sections.yml#L56-L60