Add function select_many.

[bebeka]

@example select_many('id', 'name', 'username');

[treffynnon]

I think that this would be better dealt with as an associative array on the current select() method rather than adding a new method that cannot handle column aliases like the one in this patch.

Thanks for the patch, but unfortunately on this occasion it will not be merged into the core code.

[bebeka]

Thank you for a commit, sorry that did not answer immediately.