Warn if a configured rule is not enabled

[marcelofabri]

Fixes #1350.

Way better attempt than #1806.

[SwiftLintBot]

	1 Warning
⚠️	This PR may need tests.

	12 Messages
📖	Linting Aerial with this PR took 1.87s vs 1.89s on master (1% faster)
📖	Linting Alamofire with this PR took 3.9s vs 3.94s on master (1% faster)
📖	Linting Firefox with this PR took 12.65s vs 12.8s on master (1% faster)
📖	Linting Kickstarter with this PR took 0.21s vs 0.21s on master (0% slower)
📖	Linting Moya with this PR took 1.93s vs 1.92s on master (0% slower)
📖	Linting Nimble with this PR took 1.97s vs 1.96s on master (0% slower)
📖	Linting Quick with this PR took 0.58s vs 0.59s on master (1% faster)
📖	Linting Realm with this PR took 3.48s vs 3.5s on master (0% faster)
📖	Linting SourceKitten with this PR took 1.19s vs 1.22s on master (2% faster)
📖	Linting Sourcery with this PR took 4.72s vs 4.99s on master (5% faster)
📖	Linting Swift with this PR took 26.52s vs 27.28s on master (2% faster)
📖	Linting WordPress with this PR took 21.39s vs 21.08s on master (1% slower)

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