Add margin support to ordinal legend

[Fil]

This adds support for marginTop, marginBottom and marginRight (in pixels), but keeps the existing defaults (margin-left: 0px, margin-bottom: 0.5em;) to minimize churn. We might want to revisit at a later time and support both numbers (as pixels) and lengths (as strings).

Contributed by @dgwyer

supersedes #1082 closes #1080