Use helper for context wp_title, defaulting to wp_title() suffixed by site name

Currently, there is no way to override the value displayed by `{{ wp_title }}` in a Twig template backed by a Timber context. A `wp_get_title` helper exists in `timber-helper.php` but it is not used anywhere within the Timber core.

This pull request sets the value of wp_title to the value returned by this helper, where the helper defaults to the value of `wp_title()` suffixed by the site name (`get_bloginfo('name')`). This default filter has a priority of `0` to allow easy overriding by other defined filters (`add_filter` has a default priority of `10`).