Add default render props

Moved the default implementation of rendering to default props,
reducing the branching. I couldn't change `renderInput` (custom
implementations get wrapped by a `div`, default one doesn't) and
`renderMonth` (default implementation depends on state) trivially,
so I left them out of this PR's scope.

Author: onlined

src/datetime/DateTime.js

@@ -73,7 +73,8 @@ export default class Datetime extends React.Component {
 		strictParsing: true,
 		closeOnSelect: false,
 		closeOnTab: true,
-		closeOnClickOutside: true
+		closeOnClickOutside: true,
+		renderView: ( _, renderFunc ) => renderFunc(),
 	}
 
 	// Make moment accessible through the Datetime class
@@ -123,10 +124,7 @@ export default class Datetime extends React.Component {
 	}
 
 	renderView() {
-		if ( this.props.renderView ) {
-			return this.props.renderView( this.state.currentView, this._renderCalendar );
-		}
-		return this._renderCalendar();
+		return this.props.renderView( this.state.currentView, this._renderCalendar );
 	}
 
 	_renderCalendar = () => {

src/datetime/DaysView.js

@@ -3,7 +3,8 @@ import ViewNavigation from './ViewNavigation';
 
 export default class DaysView extends React.Component {
 	static defaultProps = {
-		isValidDate: () => true
+		isValidDate: () => true,
+		renderDay: ( props, date ) => <td { ...props }>{ date.date() }</td>,
 	}
 
 	render() {
@@ -110,14 +111,8 @@ export default class DaysView extends React.Component {
 
 		dayProps.className = className;
 
-		if ( this.props.renderDay ) {
-			return this.props.renderDay(
-				dayProps, date.clone(), selectedDate && selectedDate.clone()
-			);
-		}
-
-		return (
-			<td { ...dayProps }>{ date.date() }</td>
+		return this.props.renderDay(
+			dayProps, date.clone(), selectedDate && selectedDate.clone()
 		);
 	}
 

src/datetime/YearsView.js

@@ -2,6 +2,10 @@ import React from 'react';
 import ViewNavigation from './ViewNavigation';
 
 export default class YearsView extends React.Component {
+	static defaultProps = {
+		renderYear: ( props, year ) => <td { ...props }>{ year }</td>,
+	};
+
 	render() {
 		return (
 			<div className="rdtYears">
@@ -66,18 +70,10 @@ export default class YearsView extends React.Component {
 
 		let props = {key: year, className, 'data-value': year, onClick };
 
-		if ( this.props.renderYear ) {
-			return this.props.renderYear(
-				props,
-				year,
-				this.props.selectedDate && this.props.selectedDate.clone()
-			);
-		}
-
-		return (
-			<td { ...props }>
-				{ year }
-			</td>
+		return this.props.renderYear(
+			props,
+			year,
+			this.props.selectedDate && this.props.selectedDate.clone()
 		);
 	}