Add command to generate rules documentation

[marcelofabri]

Will fix #1078

This produces a markdown looking like this.

Next steps:

• Add a target on Makefile to run this command and output it to a file.
• List rules configuration. In order to do this right, we'd need to change a bit the RuleConfiguration protocol to provide more information about the parameters.
• Add a detailedDescription: String? on RuleDescription that would be used to generate this. All descriptions currently were thought for console and Xcode usage and we should go into more details in the docs. @jpsim did an awesome job on the 0.14 release blog post that could be used as an inspiration.
• Add a Danger check to validate that the markdown was updated if a new rule is being created.

[SwiftLintBot]

	2 Warnings
⚠️	Big PR
⚠️	This PR may need tests.

	12 Messages
📖	Linting Aerial with this PR took 0.36s vs 0.34s on master (5% slower)
📖	Linting Alamofire with this PR took 2.31s vs 2.28s on master (1% slower)
📖	Linting Firefox with this PR took 10.03s vs 9.98s on master (0% slower)
📖	Linting Kickstarter with this PR took 14.86s vs 14.35s on master (3% slower)
📖	Linting Moya with this PR took 0.67s vs 0.65s on master (3% slower)
📖	Linting Nimble with this PR took 1.33s vs 1.3s on master (2% slower)
📖	Linting Quick with this PR took 0.44s vs 0.43s on master (2% slower)
📖	Linting Realm with this PR took 2.15s vs 2.0s on master (7% slower)
📖	Linting SourceKitten with this PR took 0.87s vs 0.86s on master (1% slower)
📖	Linting Sourcery with this PR took 2.81s vs 2.67s on master (5% slower)
📖	Linting Swift with this PR took 9.87s vs 9.5s on master (3% slower)
📖	Linting WordPress with this PR took 10.03s vs 9.79s on master (2% slower)

Generated by 🚫 Danger

[codecov-io]

Codecov Report

Merging #1107 into master will decrease coverage by 0.7%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1107      +/-   ##
==========================================
- Coverage   87.96%   87.25%   -0.71%     
==========================================
  Files         197      198       +1     
  Lines        9761     9840      +79     
==========================================
  Hits         8586     8586              
- Misses       1175     1254      +79

Impacted Files	Coverage Δ
Source/swiftlint/main.swift	0% <0%> (ø)	⬆️
Source/swiftlint/Commands/RulesDocsCommand.swift	0% <0%> (ø)

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[ghost]

It would also be good if this generated a Table of Contents for the top of the Markdown file. As we keep adding rules, this is going to be become one loooooong file.

[marcelofabri]

We probably should use Sourcery for this instead

[marcelofabri]

We probably should use Sourcery for this instead

Actually, that won't work 😅

[marcelofabri]

🚢

[jpsim]

Oh my, I'm reviewing this now finally and this is soooo cool. This is a very nice addition to the project.

I'm very happy you did this. I was expecting to have more nitpicks, but really I just have #1660.

[marcelofabri]

🎉

I hope we can inprove this even more in the future with rule configurations and better descriptions for rules 🙌