#13: Fix watch functionality to work in parallel

This is a follow-up to #48, capturing the full changeset.

@markdboyd, Ccing you here for the continued discussion.
Okay, for some reason I thought the watch implementation in theme.js had been removed. Looking closer at it, it seems to address my point of probably wanting more granular watch steps.

In order to avoid load sequencing complexity, it might make sense to configure the `watch-theme` task inside theme.js, and set each of the watch tasks defined there as distinct parallel watch steps.

Does that make sense?