Nested link references definitions can be used at the top-level

[dillonkearns]

Link references affect the entire document, not just the container in which they are defined

https://github.com/dillonkearns/elm-markdown/blob/master/test-results/failing/GFM/Link%20reference%20definitions.md#example-187

See also the rest of the failing cases in that file for context.

Here's where the link reference definition parsing is done and the definitions are added to the dict:

elm-markdown/src/Markdown/Parser.elm

Lines 696 to 698 in c76fd35

	, LinkReferenceDefinition.parser
	|> Advanced.backtrackable
	|> map (\reference -> Loop (addReference revStmts reference))

And here's where the inline parsing is done and receives the dict of reference definitions:

elm-markdown/src/Markdown/Parser.elm

Lines 152 to 153 in 9158bea

	Markdown.InlineParser.parse mappedReferencesDict unparsedInlines
	|> List.map mapInline

This is likely a more challenging fix, so not as well suited to a first-time contribution.