So I've just created a plugin for wintersmith, aimed at utilising my bower_components or node_modules directory from outside my contents directory. Really though, you could use any directory as an additional source of content for wintersmiths content tree.

/**
 *
 * @param {string} name
 * @see {@link https://google.com}
 */
export function foo() {
  console.log('hewwo'); 
  console.log('hello'); 
}

me@machine:~/Projects/this/one $ mythical-ls
	bower_components
	node_modules
	src
		assets
			scss
		config
		contents
			articles
			etc
			...
			index.md
		plugins
		templates
		tests
		app.coffee
	CNAME
	Gruntfile.coffee
	bower.json
	package.json
	readme.md

At first I was just putting bower_components directly inside my project/src/contents/assets/vendor/ and while this might be tenable for bower, it wasn't clean, you might come up with other reasons.

npm install wintersmith-mounter

  ...
 
  mounter:
    mounts:
      '/assets/vendor/':
        src: './bower_components'
 
  ...