Fix for register mutation in case username not included

[imsheldon]

If you omit the username field from 'REGISTER_MUTATION_FIELDS': ["email", "first_name"], and set username with signal, the login fails after registration, since we don't have username detail. This is an attempt to fix the same if username is generated with the help of some signal.

[codecov-io]

Codecov Report

Merging #81 into master will decrease coverage by 0.12%. The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master      #81      +/-   ##
==========================================
- Coverage   92.52%   92.39%   -0.13%     
==========================================
  Files          11       11              
  Lines         682      684       +2     
==========================================
+ Hits          631      632       +1     
- Misses         51       52       +1     

Impacted Files	Coverage Δ
graphql_auth/mixins.py	90.15% <50.00%> (-0.25%)	:arrow_down:

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[PedroBern]

Hi @imsheldon, thanks for the PR! I see your point and agree with the changes, but I think it would be better if we toggle this option as a new boolean flag setting, so everybody could be aware of this, preventing unaware registering without a username. Can you make this?

Here is the summary:

1. Create a new boolean setting, defaulting to false, something like HANDLED_USERNAME, or whatever you find more appropriate.
2. Check for the setting and proceed with the code.
3. Update the docs for the new setting.

[imsheldon]

Ok, I will work on it and create the PR.