Addon: Added google analytics

[pksunkara]

Fixes #3944

This PR adds a google analytics addon. This is not ready to be merged. Just a call for review for now.

TODO

• Add integration tests
• List the addon in docs

[storybook-safe-bot]

	Fails
🚫	PR is marked with "do not merge", "in progress" labels.

Generated by 🚫 dangerJS

[ndelangen]

Is this a global variable?

Might want to make that explicit:

window.STORYBOOK_GA_ID = UA-000000-01;

[ndelangen]

So I don't think we want this in any example we have.. since really I don't fancy getting into tracking people who browser our examples.

So maybe add some unit test for this?

[Hypnosphi]

Maybe a separate examples/google-analytics?

[informa]

Awesome work, is this addon nearly finished?

[pksunkara]

Need some docs work and tests.

[ndelangen]

I actually think the code is simple enough to not have any tests tbh.

Let's add some documentation and merge, so we can announce this with 4.0.0 @pksunkara

[codecov]

Codecov Report

Merging #4138 into master will decrease coverage by 0.05%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #4138      +/-   ##
==========================================
- Coverage   36.07%   36.02%   -0.06%     
==========================================
  Files         556      557       +1     
  Lines        6633     6643      +10     
  Branches      868      869       +1     
==========================================
  Hits         2393     2393              
- Misses       3796     3805       +9     
- Partials      444      445       +1

Impacted Files	Coverage Δ
addons/google-analytics/src/register.js	0% <0%> (ø)

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[pksunkara]

@ndelangen This PR is ready to go.

[igor-dv]

It should be added in https://github.com/storybooks/storybook/blob/master/ADDONS_SUPPORT.md
and in https://github.com/storybooks/storybook#addons