Ignore SwiftLint commands in file_header rule

[marcelofabri]

Fixes #1810

[marcelofabri]

This will probably have the side effect that you can't enforce/forbid a command in the beginning of the file with this rule.

[SwiftLintBot]

	12 Messages
📖	Linting Aerial with this PR took 0.37s vs 0.35s on master (5% slower)
📖	Linting Alamofire with this PR took 2.66s vs 2.52s on master (5% slower)
📖	Linting Firefox with this PR took 10.69s vs 10.35s on master (3% slower)
📖	Linting Kickstarter with this PR took 16.87s vs 16.15s on master (4% slower)
📖	Linting Moya with this PR took 1.27s vs 1.21s on master (4% slower)
📖	Linting Nimble with this PR took 1.54s vs 1.49s on master (3% slower)
📖	Linting Quick with this PR took 0.48s vs 0.46s on master (4% slower)
📖	Linting Realm with this PR took 2.42s vs 2.26s on master (7% slower)
📖	Linting SourceKitten with this PR took 0.9s vs 0.86s on master (4% slower)
📖	Linting Sourcery with this PR took 3.95s vs 3.83s on master (3% slower)
📖	Linting Swift with this PR took 11.13s vs 10.98s on master (1% slower)
📖	Linting WordPress with this PR took 10.13s vs 9.78s on master (3% slower)

Generated by 🚫 Danger

[codecov-io]

Codecov Report

Merging #1816 into master will decrease coverage by <.01%.
The diff coverage is 90.19%.

@@            Coverage Diff             @@
##           master    #1816      +/-   ##
==========================================
- Coverage   88.65%   88.65%   -0.01%     
==========================================
  Files         236      236              
  Lines       11533    11541       +8     
==========================================
+ Hits        10225    10232       +7     
- Misses       1308     1309       +1

Impacted Files	Coverage Δ
...wiftLintFramework/Rules/LetVarWhitespaceRule.swift	89.2% <ø> (ø)	⬆️
.../SwiftLintFrameworkTests/FileHeaderRuleTests.swift	100% <100%> (ø)	⬆️
...SwiftLintFramework/Extensions/File+SwiftLint.swift	93.36% <100%> (+0.03%)	⬆️
...urce/SwiftLintFramework/Rules/FileHeaderRule.swift	91.93% <85.18%> (-1.51%)	⬇️
Tests/SwiftLintFrameworkTests/TestHelpers.swift	72.95% <90%> (+0.85%)	⬆️

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[marcelofabri]

@jpsim Do you have any thoughts on this side effect?