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.
Someone else can comment on the design. My only comment is that is must have a focus style for accessibility.