tweak CSS transformation, fixes #1558

This changes the regex in [css/transform.js](https://github.com/ractivejs/ractive/blob/dev/src/config/options/css/transform.js#L3) to accommodate selector modifiers like `nth-child(2n+1)`. While I was at it I changed a couple of other things:

* `data-rvcguid` is now `data-ractive-css` - this should make it much less mysterious for anyone seeing it in their devtools
* rather than using globally unique IDs, it just uses a simple counter - the guid thing was supposed to prevent anyone from mucking about with the encapsulated styles, but a) it doesn't actually do that (you can just target `[data-rvcguid]`, you don't need to specify a value), b) so what if someone does?, and c) we can now get rid of the unused `getGuid()` module