Add parameter to redirect logged in users in [lifterlms_login] shortcode

When a non-logged in user visits a page with this shortcode, they're greeted with a login form and redirected to an admin selectable url on completing login.

Users already logged in see nothing and that's a little kink in the user experience. This can be potentially improved in two ways (as per user feedback):

1. Use the existing `redirect` parameter and redirect logged in users there.
2. Create a new parameter called `redirect_logged_in` that can be used to set a URL where logged in users are redirected when they encounter a page with the shortcode.