Add lodash (as npm dependency)

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Further flesh out node/Rails plugin integration

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Add Rails plugin dirs to Webpack modulesDirectories

This should allow Webpack to bundle assets from Rails plugins/gems,
as if they were npm modules or Bower components, but without us
having to add an entry to core's `package.json`/`bower.json`.

Signed-off-by: Alex Coles <alex@alexbcoles.com>