Add discord integration

[rennerocha]

I am creating this PR to finish the work started in #280

[codecov]

Codecov Report

Merging #348 (c40c40e) into master (a5d32c8) will decrease coverage by 0.53%.
The diff coverage is 49.23%.

@@            Coverage Diff             @@
##           master     #348      +/-   ##
==========================================
- Coverage   74.57%   74.03%   -0.54%     
==========================================
  Files          70       72       +2     
  Lines        3032     3097      +65     
  Branches      467      476       +9     
==========================================
+ Hits         2261     2293      +32     
- Misses        705      738      +33     
  Partials       66       66              

Impacted Files	Coverage Δ
spidermon/contrib/actions/discord/notifiers.py	0.00% <0.00%> (ø)
spidermon/contrib/actions/discord/__init__.py	76.19% <76.19%> (ø)

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