Customize split layout

It would be really cool if you could specify the grid layout. Currently it seems that the layout will always be a max of 2 rows and balance the number of columns. 

It would be nice to be able to specify something like:
```./terminator-split 12 --rows 4```
or 
```./terminator-split 12 --columns 2```
or
```./terminator-split --rows 4 --columns 6```