Add event tag to gtag for enhanced ecommerce google analytics

[likecodingloveproblems]

as mentioned in #185 i modify google_analytics_gtag.py to get set event data in gtag.
hopefully be useful

[codecov]

Codecov Report

Merging #186 (92be4c6) into master (896a191) will decrease coverage by 0.39%.
The diff coverage is 62.50%.

@@            Coverage Diff             @@
##           master     #186      +/-   ##
==========================================
- Coverage   94.25%   93.85%   -0.40%     
==========================================
  Files          29       29              
  Lines        1270     1285      +15     
==========================================
+ Hits         1197     1206       +9     
- Misses         73       79       +6     

Impacted Files	Coverage Δ
analytical/templatetags/google_analytics_gtag.py	81.81% <62.50%> (-11.29%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 896a191...fbe90df. Read the comment docs.

[bittner]

Please add at least one test for the new functionality in the related unit test module.

From the test the expected outcome (the resulting JavaScript code) should be clear.

[bittner]

Please rebase your PR and add a test for the business logic you added. Thank you!

[bittner]

This doesn't look like a valid pattern. The try-block shouldn't be use for testing on a data type. And what's code in the except-clause? I think this is an edge case of a "pythonic try-except" where the pattern doesn't fit.

It's not clear at first sight what we're trying to achieve. Maybe an if instanceof(value, dict) would be better here, because it's more explicit for the use case.