Small update

Set bootstrap view, add/edit languages, clean style.css, add subpage,
when word not found on language file then show word "definition", change
<code>get_template_path</code> to <code>template_path</code>
While you use here `\core\language` with `\` in beginning, in [header.php](https://github.com/simple-mvc-framework/v2/pull/97/files#diff-0bd9b71274d9716a939d7a9bd7c75917R13) in your diff, there's no `\`.
I suggest you to remove `\` in templates, because it would be more consistent.