Add option to specify cache settings per each exported block

Because module uses its own 'delivery callback' - default 'page cache' doesn't work for it.
The only cache that implies is 'block cache'.

Some blocks (for example Recent poll) have cache settings that makes export for anonymous users useless (all users get the same content though it should be user-specific).

So it would be nice to be able to override cache settings for each exported block (at least ability to disable block cache at all).

For block_export_widget_block_export_widget_format_default() and block_export_widget_block_export_widget_format_block_content() this could be done with following code:

<?php
  $block->cache = DRUPAL_NO_CACHE;
?>