Expose defaults for panel widthss

[JonDum]

Currently, defaults for sizes are hardcoded in, but it'd be nice to be able to specify the defaults for all panels for situations where you want the same defaultMinSize for all panels but you don't know the number of panels ahead of time and don't want to keep re-creating the panelWidths array.

https://github.com/DanFessler/react-panelgroup/blob/master/src/PanelGroup.js#L81-L84

      const defaultSize = 256;
      const defaultMinSize = 48;
      const defaultMaxSize = 0;
      const defaultResize = 'stretch';

to

      const defaultSize = this.props.defaultSize || 256;
      const defaultMinSize = this.props.defaultMinSize || 48;
      const defaultMaxSize = this.props.defaultMinSize || 0;
      const defaultResize = 'stretch';