Provide an actual hook_features_pipe_alter() for general altering.

A module that works based on all entities like Metatag and Pathauto (in the future) operate on an entity basis not on individual entity types. Making those modules support features by having to implement each component pipe alter will mean eventually some entity types that could be exported will not be supported. Let's use drupal_alter()'s multi-$base ability to actually run a general alter hook.