Add rvt-button, rvt-button-group, rvt-margin

[basham]

Change:

1. Add custom elements: <rvt-button>, <rvt-button-group>, <rvt-badge>
2. Add custom attribute: rvt-margin (medium spacing only so far)

Note:

1. This work is not meant to be comprehensive but rather as a discussion point. I want to also use it to illustrate our explorations for the upcoming Dev CoP meeting.
2. If this is not the direction we go, we can also remove it before 3.0.0 is officially released.
3. If we generally think this is worthy of continuing, I can try to make custom element equivalents for all the components we're working on, in parallel with the traditional BEM convention.
4. Best practice is to isolate all custom attributes under data-, but data-rvt-margin seems excessively long. I think our rvt- namespace is sufficient.
5. Custom attributes on custom elements do not need a namespace, as the element is already namespaced.
6. Because "style" is a reserved attribute name, I categorized strong/normal/subtle as "tone". In terms of "voice and tone" usage: Voice is what is said (the text content). Tone is how it is said (the visual/auditory expression and emphasis).
7. I was unsure about how I'd like nesting <button> in a <rvt-button> — but I like the style. It promotes composition over inheritance and isolates concerns.
8. Migrating away from BEM-style modifier classes to attributes will make make it easier to document exactly what each custom element offers. "'Medium' is a 'size' option. 'Strong' is a 'tone' option."
9. See article: This website has no class
10. See article: CSS Classes considered harmful