Feature/#1029 add emptyTagsToSelfClosing option to xhtml formatter

[Aizeren]

Types of Changes

Prerequisites

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• I have read the CONTRIBUTING document
• My code follows the code style of this project

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• Bug fix (non-breaking change which fixes an issue, please reference the issue id)
• New feature (non-breaking change which adds functionality, make sure to open an associated issue first)
• Breaking change (fix or feature that would cause existing functionality to change)
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• All new and existing tests passed

Description

Add emptyTagsToSelfClosing option to XhtmlMarkupFormatter
Resolve #1029

[FlorianRappl]

LGTM - thanks!