alter hook for term_permissions_allowed()

Hi,

This module works great but I need to extend it for a specific business case. So I created a hook_alter function rather than inject my requirements into this module where they would not be appropriate.

With this patch, a third party module can alter the term permissions by calling 'hook_term_permissions_allowed_alter'. Only return a value if you want to alter the term permissions.

Thanks,
Rim