Add Twig filter for date formatting

Problem/Motivation

If you have a date variable in Twig you can't format it using date formats you've created in Drupal. Twig has a date filter but it does not hook up to Drupal things.

Beta phase evaluation

Proposed resolution

Add a new filter to do this.

Before

In preprocess or the render array variables.

// Functional service wrapper from D7.
format_date($timestamp, 'html_date');
// Date Formatter Service
\Drupal::service('date.formatter')->format($timestamp, 'html_date');

After

{{ timestamp|format_date('html_date') }}

Also note that Twig has a date filter & function already, but this doesn't work with the named formats defined in the Drupal UI or system
http://twig.sensiolabs.org/doc/filters/date.html
{{ post.published_at|date("m/d/Y") }}

Remaining tasks

TBD

User interface changes

n/a

API changes

API addition, ability to use Drupal date formats from Twig templates.

Data model changes

None.