add func renderHeaderColumn

examples/basic.js

 import React, {PureComponent} from 'react';
 import ReactDOM from 'react-dom';
 import ReactFinder from '../src';
-import {mockTree, findInTree, atTreePath} from '../src/utils';
+import { mockTree, findInTree, atTreePath } from '../src/utils';
 
 const mockData = mockTree(3);
 
@@ -14,19 +14,19 @@ class Test extends PureComponent {
     }
 
     render() {
-        const {dataSource} = this.state;
+        const { dataSource } = this.state;
         return (
             <div>
                 <ReactFinder
-                    renderHeaderColumn={({parentOpenedKey, openedKey, selectedKey}) => (<div>123</div>)}
+                    renderHeaderColumn={({ parentOpenedKey, openedKey, selectedKey }) => (<div>header</div>)}
                     dataSource={dataSource}
-                    renderItem={({data, isLeaf}) => (
+                    renderItem={({ data, isLeaf }) => (
                         <span>{data.key} {!isLeaf && <span style={{float: 'right'}}>></span>}</span>
                     )}
                     selectedKey={this.state.selectedKey}
-                    onSelect={(selectedKey, {data}) => {
-                        this.setState({selectedKey, selectedData: data});
-                        const nodeData = findInTree(dataSource, (node) => node.key === selectedKey, {withAppendData: true});
+                    onSelect={(selectedKey, { data }) => {
+                        this.setState({ selectedKey, selectedData: data });
+                        const nodeData = findInTree(dataSource, (node) => node.key === selectedKey, { withAppendData: true });
                         if (nodeData) {
                             console.log('selected node', atTreePath(dataSource, nodeData.loc));
                         }