Add an Accessibility Category

[tnoetzel]

Is your feature request related to a problem? Please describe. I do a lot of accessibility work with one of my clients, and we've developed several custom checks for accessibility. It would be nice to be able to isolate these from other issues when we run theme-check

Describe the solution you'd like Would love to be able to tag these checks as either accessibility or a11y and then run something like shopify theme check --category a11y and only have those checks run