Allow nested Raw queries

For context, we wanted to run postgis queries with raw expressions being generated from different parts of the codebase. Stuff that boiled down to things like:

```javascript
.where(knex.raw('ST_Distance_Sphere(?, coordinate) < ?', [
  knex.raw('ST_SetSRID(ST_MakePoint(?, ?)', [-80, 150]),
  1000
]));
```

Manually combining the query strings every time is pretty horrible :p 

But yea, I think splicing them up in the constructor is a fairly reasonable solution to nested raws. 

Thoughts?