Writing require modules when running in `r.js`

Adding ability to write require modules that will load CSS content into style nodes

https://github.com/guybedford/require-css/issues/169
@MiguelCastillo -- I think you may need to add a `return writeCss;` here (after `(d.createTextNode(c));`), otherwise the value of `@writecss` will end up being undefined.

(On a side note, I just discovered this same issue last night, and finding this PR this morning has saved me countless hours of work, so thank you!)