Specify a Filter Name That Differs from the Column Name

I'd like to have a property that is filterable but have the name of the filterable property be different than the name of the column that backs the property. For example

```
type User struct {
FirstName string `json:"firstName" rql:"filter,name=firstName,column=first_name"`
}
```

So a filter like this

```
{
"filter": {
"firstName": "Bob"
}
}
```
produces

`WHERE first_name="Bob"`

I don't see support for this currently. Is that correct? If so, would doing this be that difficult? I don't know the codebase very well, but this could be really useful for my team using this library.