Allow opening brace on newline in case of multiline class or struct

[revolter]

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[SwiftLintBot]

	1 Warning
⚠️	Please include a CHANGELOG entry to credit yourself! You can find it at CHANGELOG.md.

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📖	Linting Aerial with this PR took 2.42s vs 2.49s on master (2% faster)
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📖	Linting SourceKitten with this PR took 1.21s vs 1.22s on master (0% faster)
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📖	Linting Swift with this PR took 14.63s vs 14.63s on master (0% slower)
📖	Linting WordPress with this PR took 24.02s vs 24.27s on master (1% faster)

Here's an example of your CHANGELOG entry:

* Allow opening brace on newline in case of multiline class or struct.  
  [revolter](https://github.com/revolter)
  [#issue_number](https://github.com/realm/SwiftLint/issues/issue_number)

note: There are two invisible spaces after the entry's text.

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