Support custom dimensions in GA pageviews and events

[ttmarek]

Checklist

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• [x] I have added tests that prove my fix is effective or that my feature works.
• [x] I have added all necessary documentation (if appropriate)

What was done

These changes adds support for custom dimensions when tracking events and page views in Google Analytics. The API adopted here was directly taken directly from the discussion in #278.

Associated Issues

• Fixes #278

[ttmarek]

If you'd like to test these changes they are available in:

npm install @redux-beacon/google-analytics@next