Support aggregations and selecting only some fields

Now we write:
```go
	db := ctx.GetGorm().
		Select("max("+PointDBSchema.TripPointID.String()+") as max_id, "+
			"count("+PointDBSchema.PlaceID.String()+") as rating").
		Where(PointDBSchema.LocationID.String()+" = ?", locationID).
		Group(PointDBSchema.PlaceID.String()).
		Order("rating DESC").
		Limit(limit)
```