Support configurations of global check constants about gtxilib

[j-sid]

Cool, I like the idea of using AA or AAA configs I just want to ensure that we make it clear that passing AA/AAA on GTX does not mean the app is A11y ready, only means that all AA/AAA checks in GTX have passed.

[j-sid]

This is the direction we are taking as well, we plan to provide a config object that can be provided to GTX for the checks to read from and configure themselves. Please let us know if this would not work for you or have a better idea.
We will start with configs for contrast ratio and touch target size, and plan to expand it for more checks.

[niksawtschuk]

@j-sid that sounds great. I think it might be useful to have configuration "defaults" that align with a11y standards around the world. For example, there could be an easy way to quickly set the tool to use WCAG AA or AAA rules. I think this may also help clear up #1