Add the ability to control options via constants

[michaelryanmcneill]

Many times, especially in enterprise environments, administrators would rather set critical configuration options (such as those that surround authentication) via wp-config.php instead of via an options page. When this happens, the options page should have the inputs greyed out and explain that they are configured via wp-config.php.

[michaelryanmcneill]

The underlying code work is done but documentation needs to be added.

[michaelryanmcneill]

Documentation has been added to the readme.txt: https://github.com/michaelryanmcneill/shibboleth/blob/2.0-alpha/readme.txt