Blocks: Add a Definition block

[ramonjd]

This is a block I've been working on, and I thought I'd give it a home here while I cut out the rough edges.

It's a definition block, which I created primarily to highlight term definitions semantically. Similar to a quote block I suppose.

The bells and whistles include an online search, which should or should not feature in the first iteration.

More context pcjTuq-lN-p2