Alternative complex alias hook

@Suor: This way you should be able to add columns support by overriding `_aliasToString` & you wouldn't need to re-implement the _alias bits, like this:

```javascript
  Values.prototype.columns = function () {
    this._columns = true;
    return this;
  }
  Values.prototype._aliasToString = function (opts) {
    var alias = Values.super_.prototype._aliasToString.apply(this, arguments);
    if (!alias || !this._columns)
      return alias;
    
    var cols = _.keys(this._values[0]).map(sql._quoteColOrTbl).join(', ');
    return alias + ' (' + cols + ')';
  }
```