Fixed issue with re-rendering a Reactable with different data

Fixed an issue where re-rendering the component with a completely different data set would result in an invalid state. Columns from the original dataset were being retained, and any new columns were being ignored.

This patch adds an update method that fetches columns from the dataset whenever the component's props change. This can be improved upon, but it fixes the issue and removes the issue of copying props(columns) from child to parent.
I'd prefer `typeof` to be called as a function here.