Added adminRole property and functionality

[pcorey]

When being used as a drop-in admin panel, sometimes the existing system isn't using the normal admin role to identify administrators. In my case, Admin was being used, but it's feasible that other roles could be used as well.

This PR adds a adminRole option to the AdminConfig. If it is set, that string is used as the "admin role" in the system. If it's not set, it defaults to admin.

I updated the docs as well.