width/height methods taking percentage parameter
Add width/height methods taking percentage parameter
* width(_ percent: FPercent)
* height(_ percent: FPercent)
* minWidth(_ percent: FPercent)
* maxWidth(_ percent: FPercent)
* minHeight(_ percent: FPercent)
* maxHeight(_ percent: FPercent)
Usage examples:
* view.flex.width(50%)
* view.flex.height(25%)
* Added by [Luc Dion](https://github.com/lucdion) in Pull Request [#28](https://github.com/lucdion/FlexLayout/pull/28)