Text search

Adding text indexes and score sort.

There was a comment on Sort implying the following syntax:
```go
query4 := collection.Find(nil).Sort("score:{$meta:textScore}")
```
but I think it makes more sense to be coherent with the index syntax and do:
```go
query4 := collection.Find(bson.M{"$text": bson.M{"$search": "some text"}})
                    .Select(bson.M{"score": bson.M{"$meta": "textScore"}})
                    .Sort("$score:textScore")
```
Does that make sense?

It might be missing documentation. But I want to see if this might get accepted.
Please use standard Go names for the fields (e.g. DefaultLanguage) and use the field tag to adjust the name as necessary (see ExpireAfter above).