New: Experimental support for CSS Auron (fixes #765)

Experimental support for CSS Auron. I'm leaving out documentation for now, just in case we decide this isn't working out. Waiting on some additional documentation for CSS Auron, but the basic idea is that inside of a rule you can do:

```js
if (context.matches(node, "FunctionExpression Identifier")) {
    // ...
}
```

So it doesn't replace the current way of traversing the tree, but augments what we already have to hopefully help make the code inside of rules a bit easier to deal with.

The big thing holding back further testing is the lack of documentation related to patterns. I've filed https://github.com/chrisdickinson/cssauron-esprima/issues/1 requesting that.