This adds the ability for the style-loader to re-use a single <style>
element reference when adding/removing styles from the page. This is
particularly important when you want to use style-loader with a browser
like IE9, which enforces a hard-limit on the # of style tags allowed on
the page at any given point.
This is added as a new option `singleton`, and is automatically turned
on if the browser is determined to be IE9.
This ensures that items with the same id are only added once.
i. e. multiple css file and `@import` the same base css file and it's only added once to the DOM.
webpack/css-loader#17