Add a way to alter hook_commons_entity_integration() implementations.

Would be nice if commons fields (like Radioactivity and Follow) offer a way to disable them on existing features.

For instance, take the Poll module:

<?php/**
 * Implements hook_commons_entity_integration.
 */
function commons_polls_commons_entity_integration() {
  return array(
    'node' => array(
      'poll' => array(
      ),
    ),
  );
}}?>

Currently, there is no way to change this to the following without changing the module:

<?php/**
 * Implements hook_commons_entity_integration.
 */
function commons_polls_commons_entity_integration() {
  return array(
    'node' => array(
      'poll' => array(
        'exclude_commons_follow' => TRUE,
      ),
    ),
  );
}?>

This could be achieved by giving modules the change to alter the hook_commons_entity_integration outcome by changing this:

<?php
function commons_follow_flag_alter(&$flag) {
  $commons_entity_integrations = module_invoke_all('commons_entity_integration');
  ...
}
?>

to

<?php
function commons_follow_flag_alter(&$flag) {
  $commons_entity_integrations = module_invoke_all('commons_entity_integration');
  drupal_alter('commons_entity_integration', $commons_entity_integrations);
  ...
}
?>