[FEATURE] Introduce HTML formatter

[eliashaeussler]

This PR introduces a new html formatter. It generates an HTML report that looks like this:

The report can be generated like follows:

composer update-check -f html > update-check-report.html

Resolves: #146

[coveralls]

Pull Request Test Coverage Report for Build 11540178784

Details

• 0 of 19 (0.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 20.373%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/IO/Formatter/FormatterFactory.php	0	1	0.0%
src/IO/Formatter/HtmlFormatter.php	0	18	0.0%
<!--	Total:	0	19	0.0%	-->

Totals
Change from base Build 11539824217:	-0.2%
Covered Lines:	382
Relevant Lines:	1875

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💛 - Coveralls

[eliashaeussler]

I'll probably rewrite this to a custom reporter instead of a formatter. Please hang on, @mteu.