add username option to redis config

[erdos4d]

When setting DEFENDER_REDIS_URL, add capability to use non-default username.

[codecov]

Codecov Report

Merging #218 (eab8a1e) into master (0b96c53) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #218      +/-   ##
==========================================
+ Coverage   92.46%   92.50%   +0.03%     
==========================================
  Files          13       13              
  Lines         398      400       +2     
==========================================
+ Hits          368      370       +2     
  Misses         30       30              

Impacted Files	Coverage Δ
defender/connection.py	96.77% <100.00%> (+0.22%)	⬆️

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[aleksihakli]

LGTM, thank you for the PR :) @kencochrane is the person who can make a new version release to PyPI for django-defender.

[kencochrane]

It should be released to pypi. Thanks!