Support for <AutoLink> and link reference definition associations.

[pcj]

• An abstract node 'Reference' that captures the link label, title,
  description attributes for subclasses Link, Image, AutoLink.
• AutoLink node class that allows Visitor to differentiate between
  inline links and types.
• Support in the InlineParserImpl to capture the association between
  links and link reference definitions.
• static Debugging functions

[pcj]

Looking into that test failure.

[robinst]

It makes sense that AutoLink is a separate node type.

I'm not sure about subclassing Link though. After all, it doesn't have all the attributes that a Link has (no label or title). Have you thought about having it extend Node instead?

[robinst]

Looks like a good change, but I'm not sure about the class structure yet.

Do you think it would make sense to have [foo]: /url "title" be its own Node type, like ReferenceDefinition? Something like this maybe:

• [foo](/uri "title") is a Link, which has getDestination, getTitle (not sure about getLabel)
• [foo][foo], [foo][] and [foo] are ReferenceLink extends Link, which have an additional getDefinition which returns the ReferenceDefinition

Does that make sense?

[robinst]

I've created an issue for this improvement, for which this PR can be a starting point: #98. Closing this in the meantime.