fix #63 (part1 - cell editing validation)

* implement cell editing validation

* add test cases for cell editing validation

* add story for validator for cell editor

* add docs for cell editor validation

packages/react-bootstrap-table2-example/examples/cell-edit/cell-edit-validator-table.js

@@ -0,0 +1,85 @@
+import React from 'react';
+/* eslint no-unused-vars: 0 */
+import { BootstrapTableful } from 'react-bootstrap-table2';
+import Code from 'components/common/code-block';
+import { productsGenerator } from 'utils/common';
+
+const products = productsGenerator();
+
+const columns = [{
+  dataField: 'id',
+  text: 'Product ID'
+}, {
+  dataField: 'name',
+  text: 'Product Name'
+}, {
+  dataField: 'price',
+  text: 'Product Price',
+  validator: (newValue, row, column) => {
+    if (isNaN(newValue)) {
+      return {
+        valid: false,
+        message: 'Price should be numeric'
+      };
+    }
+    if (newValue < 2000) {
+      return {
+        valid: false,
+        message: 'Price should bigger than 2000'
+      };
+    }
+    return true;
+  }
+}];
+
+const sourceCode = `\
+const columns = [{
+  dataField: 'id',
+  text: 'Product ID'
+}, {
+  dataField: 'name',
+  text: 'Product Name'
+}, {
+  dataField: 'price',
+  text: 'Product Price',
+  validator: (newValue, row, column) => {
+    if (isNaN(newValue)) {
+      return {
+        valid: false,
+        message: 'Price should be numeric'
+      };
+    }
+    if (newValue < 2000) {
+      return {
+        valid: false,
+        message: 'Price should bigger than 2000'
+      };
+    }
+    return true;
+  }
+}];
+
+const cellEdit = {
+  mode: 'click',
+  blurToSave: true
+};
+
+<BootstrapTableful
+  keyField='id'
+  data={ products }
+  columns={ columns }
+  cellEdit={ cellEdit }
+/>
+`;
+
+const cellEdit = {
+  mode: 'click',
+  blurToSave: true
+};
+export default () => (
+  <div>
+    <h3>Product Price should bigger than $2000</h3>
+    <BootstrapTableful keyField="id" data={ products } columns={ columns } cellEdit={ cellEdit } />
+    <Code>{ sourceCode }</Code>
+  </div>
+);