Don't allow to place items that exceed maxRows

Hi,

Thanks for the awesome library.

I'm looking for a way to limit the rows in which the items can be placed.
It works horizontally (limit is set with the ```columns``` variable) but not vertically (```maxRows``` is ignored and I can place the item on any row I choose).

In this pull request I've added a quick check for vertical placing just like the horizontal one has, but it seems to break when ```pushing``` is set to true. Before making it work with pushing I wanted to check with you:

Is there a different way to accomplish this that I'm missing?
Would you be interested in adding this functionality?
Should there be an additional setting variable that controls this functionality? Something like ```limitRows```?