Add prohibited_nan_comparison opt-in rule

[marcelofabri]

Fixes #2086

[SwiftLintBot]

	12 Messages
📖	Linting Aerial with this PR took 1.3s vs 1.25s on master (4% slower)
📖	Linting Alamofire with this PR took 2.33s vs 2.27s on master (2% slower)
📖	Linting Firefox with this PR took 9.77s vs 9.43s on master (3% slower)
📖	Linting Kickstarter with this PR took 16.1s vs 15.66s on master (2% slower)
📖	Linting Moya with this PR took 1.22s vs 1.22s on master (0% slower)
📖	Linting Nimble with this PR took 1.44s vs 1.4s on master (2% slower)
📖	Linting Quick with this PR took 0.56s vs 0.55s on master (1% slower)
📖	Linting Realm with this PR took 2.52s vs 2.47s on master (2% slower)
📖	Linting SourceKitten with this PR took 1.09s vs 1.07s on master (1% slower)
📖	Linting Sourcery with this PR took 7.2s vs 6.99s on master (3% slower)
📖	Linting Swift with this PR took 13.2s vs 12.98s on master (1% slower)
📖	Linting WordPress with this PR took 16.02s vs 15.81s on master (1% slower)

Generated by 🚫 Danger