search parameters overrideable

Code is a little bit convoluted (owing to the fact that search config is not just a flat object), but the end effect is that passing in a third parameter of e.g. 

```
{
  highlight: true, 
  facets: {
    minThreshold: 50, 
    names: ["regions']
  }
}
```
will override as you'd expect it to.

In addition if the key `names` was replaced with `'+names'` then the array woudl be concatenated with the default list of names.

If this looks good I'll write tests and document properly

@richard-still-ft