Allow access_rule to be a Callable

For custom authentication etc (if a user has to have a specific role for example):

``` php
'access_rule' => function() {
if ( is_super_admin() ) {
return false;
}

global $wp;
return wp_login_url( add_query_arg( $wp->query_string, '', home_url( $wp->request ) ) );
}
```

Locate the access_rule handling in HM_Rewrite and trace how rewrite access rules are evaluated. Add support for a callable access_rule matching the issue's example, then verify that callable rules can allow or redirect access without breaking existing rule behavior.

Written by the indexing model from the issue text.