Missing allowResize prop

[Roshdy]

I know this is based on the react-split-panes which is great. But this one is missing a key feature allowResize=false

The workaround I'm guessing would be stopping the event onDragStart, but the resizer would it still be there, which will make us forced to use conditional styling.

Also if possible to include percentage size as well, as the workaround now it to manually calculate it based on the window width/height