renderer: add support for extending node renderers

[benclmnt]

Hi, I added support for extending node renderer similar to goldmark's HTML renderer.

An example usage:

markdownRenderer := markdown.NewRenderer()
markdownRenderer.AddOptions(
	renderer.WithNodeRenderers(util.Prioritized(&latex.BlockRenderer{
		RendererType: latex.RendererTypeMarkdown,
	}, 502)),
)
md := goldmark.New(
	goldmark.WithRenderer(markdownRenderer),
)	

[coveralls]

Pull Request Test Coverage Report for Build 7825492430

• 0 of 102 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 7551523042:	0.0%
Covered Lines:	452
Relevant Lines:	452

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💛 - Coveralls

[teekennedy]

Other than my one requested change this looks good!