Add ConfigureNewLineExtension markdown extension

[augustoproiete]

This PR adds an extension which allows devs to define what string to use for line-endings when writing.

As per my comment in #211, Markdig always uses \n for line-breaks. This extension exposes an easy way to override the default behavior when defining the pipeline.

Below is an example on how to define that line-endings should use Environment.NewLine:

var pipeline = new MarkdownPipelineBuilder()
	.ConfigureNewLine(Environment.NewLine)
	.UseAdvancedExtensions()
	.Build();

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ps: This PR builds on #213, which should be merged first.

[xoofx]

Thanks! Could you add a few tests?

[augustoproiete]

Thanks! Could you add a few tests?

Tests added via #222