Add rs-link class

In many applications, it's desirable to have a button that's styled like a link. Canon allows this via the rs-btn-link class, but that classes is tailored for buttons and aligns with other buttons. Links within text or tables that use the rs-btn-link class will be subtly misaligned. This pull request adds a class called rs-link, which styles elements to appear like links, minus the alignment associated with rs-btn-link.
Don't remove default focus styles unless you replace it with something else. I can't remember if an `A` tag has focus styles defined. If so, please make it match. Otherwise, please leave the default styles.