Fix #86: Accept both numbers and strings as PropTypes for width and height

[bausa]

Fix #86

Accept both numbers and strings as PropTypes for width and height

[grean]

I would like to use a "auto" width et height, on PopupDialog component and it works fine. But prop types are number and it generates a type error.

<PopupDialog> width={'auto'} height={'auto'} ... </PopupDialog>

[jacklam718]

@grean Temporarily, you can use null