add watch mode for build-storybook

[yjcxy12]

Issue: #2762

What I did

Added -w --watch option for build-storybook.

How to test

Link storybook to an existing app, run build-storybook --watch. Watch should be enabled.

Is this testable with jest or storyshots?

No.

Does this need a new example in the kitchen sink apps?

Not sure / don't think so.

Does this need an update to the documentation?

Yes

If your answer is yes to any of these, please make sure to include it in your PR.

[codecov]

Codecov Report

Merging #2866 into master will decrease coverage by 0.07%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2866      +/-   ##
==========================================
- Coverage   35.81%   35.73%   -0.08%     
==========================================
  Files         428      428              
  Lines        9424     9444      +20     
  Branches      963      989      +26     
==========================================
  Hits         3375     3375              
- Misses       5404     5417      +13     
- Partials      645      652       +7

Impacted Files	Coverage Δ
app/vue/src/server/build.js	0% <0%> (ø)	⬆️
app/polymer/src/server/build.js	0% <0%> (ø)	⬆️
app/react/src/server/build.js	0% <0%> (ø)	⬆️
app/angular/src/server/build.js	0% <0%> (ø)	⬆️
app/vue/src/server/config/babel.js	0% <0%> (-100%)	⬇️
app/vue/src/server/utils.js	0% <0%> (-53.58%)	⬇️
...modules/ui/components/stories_panel/text_filter.js	30.98% <0%> (ø)	⬆️
lib/ui/src/libs/withLifecycleDecorator.js	17.14% <0%> (ø)	⬆️
addons/viewport/src/components/viewportInfo.js	36.36% <0%> (ø)	⬆️
addons/a11y/src/components/Report/Item.js	0% <0%> (ø)	⬆️
... and 65 more

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[igor-dv]

Everything looks reasonable to me. I will test it later today.

[igor-dv]

Tested. LGTM.