Add method for retrieving value set for a directive

[bkosborne]

The CSP builder has no method for retrieving the value that's currently set for directives. There's only a method to set the value for a directive. I have a scenario where I initialized the CSP builder from a large config array, and now I need to apply some selective additions to it, but need to get the value for some directives first.

[bkosborne]

Nevermind, I see now I can use the ->addSource method to do what I need