Add Discord integration

[anapaulagomes]

Add Discord integration. To test it you'll need two environment variables: SPIDERMON_DISCORD_WEBHOOK_URL and SPIDERMON_DISCORD_FAKE. See how to get a discord webhook URL here.

Checklist:

• [x] Add Discord integration
• [x] Add tests
• [x] Add docs

[codecov[bot]]

Codecov Report

Merging #280 (205c303) into master (f2b21e4) will decrease coverage by 0.53%. The diff coverage is 48.43%.

@@            Coverage Diff             @@
##           master     #280      +/-   ##
==========================================
- Coverage   74.21%   73.68%   -0.54%     
==========================================
  Files          68       70       +2     
  Lines        3033     3097      +64     
  Branches      462      471       +9     
==========================================
+ Hits         2251     2282      +31     
- Misses        719      752      +33     
  Partials       63       63              

Impacted Files	Coverage Δ
spidermon/contrib/actions/discord/notifiers.py	0.00% <0.00%> (ø)
spidermon/contrib/actions/discord/__init__.py	75.60% <75.60%> (ø)

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[anapaulagomes]

The build is failing due to a formatting issue that isn't related to this PR. How should I proceed?

[anapaulagomes]

Any news here @rennerocha?

[netoferraz]

Hey guys, this PR could add a very useful feature to this project. There are any intention to accept this suggestion for the project?

[rennerocha]

Yes, we want this PR to be merged as soon as possible. Probably it will be available in our next release.

[lcsvillela]

I am working at this :)

[rennerocha]

This feature will be finished in https://github.com/scrapinghub/spidermon/pull/348