Feature request: adding aXe configuration for a11y addon

[wuweiweiwu]

Issue: #2830

What I did

Added an extra export function configure to the a11y addon for users to configure aXe

usage:

import React from 'react';
import { storiesOf } from '@storybook/react';

import { checkA11y, configure } from '@storybook/addon-a11y';

const whateverConfigYouWant = {};

configure(whateverConfigYouWant);

storiesOf('button', module)
  .addDecorator(checkA11y)
  .add('Accessible', () => (
    <button>
      Accessible button
    </button>
  ))
  .add('Inaccessible', () => (
    <button style={{ backgroundColor: 'red', color: 'darkRed', }}>
      Inaccessible button
    </button>
  ));

How to test

I used the official storybook to test configuring axe.

Is this testable with jest or storyshots?
N
Does this need a new example in the kitchen sink apps?
N
Does this need an update to the documentation?
Y
If your answer is yes to any of these, please make sure to include it in your PR.

[igor-dv]

configure is a too generic name. Please rename it to the configureA11y.

[codecov]

Codecov Report

Merging #3285 into master will decrease coverage by 0.03%.
The diff coverage is 25%.

@@            Coverage Diff             @@
##           master    #3285      +/-   ##
==========================================
- Coverage   36.09%   36.05%   -0.04%     
==========================================
  Files         445      445              
  Lines        9783     9793      +10     
  Branches      902      890      -12     
==========================================
  Hits         3531     3531              
- Misses       5669     5707      +38     
+ Partials      583      555      -28

Impacted Files	Coverage Δ
addons/a11y/src/index.js	0% <0%> (ø)	⬆️
addons/a11y/src/A11yManager.js	0% <0%> (ø)	⬆️
addons/a11y/src/components/WrapStory.js	54.09% <60%> (-7.02%)	⬇️
lib/core/src/client/preview/syncUrlWithStore.js	40.9% <0%> (ø)	⬆️
addons/jest/src/components/Indicator.js	0% <0%> (ø)	⬆️
app/react/src/client/preview/error_display.js	54.54% <0%> (ø)	⬆️
lib/ui/src/libs/key_events.js	33.96% <0%> (ø)	⬆️
...ents/stories_panel/stories_tree/tree_decorators.js	34.37% <0%> (ø)	⬆️
addons/a11y/src/components/Report/Item.js	0% <0%> (ø)	⬆️
... and 63 more

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[wuweiweiwu]

@igor-dv @Hypnosphi updated :)