retain scope when including a template

[thomastrapp]

• Use data from current scope (m_data) when rendering an included template.
  This allows included templates to access loop variables, which was not possible before.
• Add test
• This fixes issue Error when accessing loop varible inside of included inja template #101.
• Commit 70096d4: The single_include-header was out of sync.

Example:

nlohmann::json data;
data["cities"] = nlohmann::json::array({{{"name", "Munich"}}, {{"name", "New York"}}});
data["locations"] = nlohmann::json::array({{{"name", "Amsterdam"}}, {{"name", "Mumbai"}}});

inja::Environment env;
env.include_template("item.tpl", env.parse("{{ loop.index1 }}: {{ item.name }}\n"));

env.render_to(
    std::cout,
    env.parse("{% for item in cities %}{% include \"item.tpl\" %}{% endfor %}"),
    data);
env.render_to(
    std::cout,
    env.parse("{% for item in locations %}{% include \"item.tpl\" %}{% endfor %}"),
    data);

// Output:
// 1: Munich
// 2: New York
// 1: Amsterdam
// 2: Mumbai

Again, if there is anything that I can do to get this merged, please let me know.
Thanks!

[pantor]

LGTM!