fix: allow override onShowMore callback (#1214)
fix: allow override onShowMore callback (#1214)

I've replaced `this._showMore` by callback from props. Seems it was added by mistake [here](https://github.com/intljusticemission/react-big-calendar/blob/f9a873368a78f5ced81b799c4dffe1095b3ab712/src/Calendar.jsx#L280), `_showMore` is always undefined.

Here it will be execute:
https://github.com/intljusticemission/react-big-calendar/blob/1d62c432eaa183ed6b38f08cfcec5ee7edcbfe41/src/Month.js#L300-L307

Related to #1147 
TeaBough/react-big-calendar,intljusticemission/react-big-calendar,jquense/react-big-calendar,jquense/react-big-calendar,jquense/react-big-calendar