Hotfix for Timber::get_posts behavior

Fixes and improvements

• Updated to most current version of Twig.
• This restores the prior behavior before #1813 / 1.9.3 when using Timber::get_posts(). This is now controllable by devs.
• Add support for non-cookied comment awaiting moderation message #1954 (thanks @codeclarified).
• Avoids a potential WSOD when incorrectly specifying template filenames #1984 (thanks @aj-adl).

Changes for Theme Developers

• If you run into problems with unknown Twig_SimpleFilter or unknown Twig_Filter classes, you can use Timber\Twig_Filter instead #1963.
• You can use WordPress's behavior of get_posts() (versus WP_Query) via a filter. By default, Timber uses the behaviors of WP_Query in Timber’s queries #1989 (thanks @palmiak).

// Turn on WordPress’s behavior of get_posts when using Timber::get_posts()
add_filter( 'timber/get_posts/mirror_wp_get_posts', '__return_true' );