Allow recipes to be triggered automatically

Here's an example of an 'update' task in Aegir, that essentially runs 'drush updatedb'. In a drush_hook_COMMAND_pre_validate() implementation, we take a backup first, so that we can restore in a drush_hook_COMMAND_pre_rollback() implementation should the update fail.

We use the pre_validate hook here to leave room for a pre_COMMAND hook implementation to come after the backup but before updatedb, so that we can (in a separate extension) run a 'git pull'. This pull request will allow for altering the order of execution of implementations of the same hook, which is currently resolved alphabetically (iirc). With that merged, we can move our backup to drush_provision_tasks_extra_pre_provision_update() and implement another drush_hook_pre_provision_update() with our git code, along with the required hook_drush_invoke_alter() to get the order right.

As for the spec, I'd suggest an element that contains a simple array of command names to react to, which would assume the 'post' hook. If running in a different hook is required, the command is replaced by an array with more specifics. Something like:

$recipe = array(
  [...]

  'commands' =>  array(
    'en',  // run after enabling a module
    'dl',  // run after downloading a module
    array(
      'command' => 'updatedb',
      'hook' => 'pre',  // run before updating the database
      'weight' => -100,  // run before other recipes in updatedb's pre hook
  )

  [...]