add possible namespacing for less/css

This would allow to have an input field for a css class on the `customize.html` of bootstrap and would add namespaces to the generates CSS like this:

```
.my-custom-ns article,
.my-custom-ns aside,
.my-custom-ns details,
.my-custom-ns figcaption,
.my-custom-ns figure,
.my-custom-ns footer,
.my-custom-ns header,
.my-custom-ns hgroup,
.my-custom-ns nav,
.my-custom-ns section {
  display: block;
}
```