Add rework-npm

Introduces the ability to inline @import statements, run conformance
checks on individual files, and to inline CSS files installed with npm.

An issue I'd like input on is how to pass the rework-npm options (or a subset) into this plugin bundle. The changeset shows how I've had to use the path from the repo root, rather than relative to the initial entry file (because rework-npm doesn't know where the entry file lives). Any ideas for a nice solution? cc @conradz

Fix gh-10