Supporting match regexps in where queries

I was working on implementing an autocomplete API, and needed this functionality. 

The idea here is if you use a query like this:

```
model.all({ where: { key: /^foo/ba.*?$/i })
```

The rethink adapter will be able to convert the regexp into the RE2 string syntax that rethink uses, and make a successful filter/match query, returning objects with keys: foo/bar, foo/baz.

Tested and working for my limited use case (matching strings that start with certain characters).