acyclic imports

[mbostock]

Fixes #659. No functional changes. To summarize:

• A new options.js takes much of mark.js to normalize input options.
• A new channels.js implements the mark sort option.
• A new dimensions.js takes the dimensions-related code out of plot.js.
• The basic transforms are combined into transforms/basic.js.
• The remainder of mark.js and facet.js are folded into plot.js.

I looked at using the eslint-plugin-import to detect this at ESLint, but it adds a bunch of dependencies and is slow and doesn’t work out of the box with module aliases. So, we should just rely on Rollup’s warnings for now.