Support for View Modes

Take a look at this article: https://www.phase2technology.com/blog/big-bad-layout

The author provides a code sample that would be very easy to implement here:

/**
* Implements hook_theme_suggestions_HOOK_alter().
*/
function mine_theme_suggestions_block_alter(array &$suggestions, array $variables) {
  // Provide suggestion for block templates by custom block type.
  if (!empty($variables['elements']['content']['#block_content'])) {
    $block = $variables['elements']['content']['#block_content'];
    // Add `block--BLOCK-TYPE.html.twig`.
    $suggestions[] = 'block__' . $block->bundle();
    $view_mode = $variables['elements']['#configuration']['view_mode'];
    if (!empty($view_mode)) {
      // Add `block--BLOCK-TYPE--VIEW-MODE.html.twig`.
      $suggestions[] = 'block__' . $block->bundle() . '__' . $view_mode;
    }
  }
}

Patch attached. I re-organized the code a bit to better accommodate the addition of the view-mode suggestion code. I also added some comments.

This patch assumes that all BlockContentInterface instances have a view mode. If this isn't the case, let me know, and I'll add a check.

The original code assumes that 'block__block_content' may be not be present as a suggestion. I left this in, even thought I'm not sure where this would occur off the top of my head.

@nerdstein - I'm going to submit another patch that also updates README.md.

Patch attached.

Had a bit of trouble testing this because none of the core D8 mechanisms for rendering blocks with alternate view modes seem to render blocks like regions do.

I was ultimately able to get this working with the "Full" view mode, which proves that it works. RTBC

If #3067045: Add classes to blocks gets committed first, then we'll need to update this patch to also add a class for the view mode.