* Add tests for backlinks
* Add backlinks indexer
* Use backlinks indexer in getTiddlerBacklinks if available
* Extract link extraction into its own method
This way we can provide an arbitrary parse tree, rather than just a
title, which will allow us to compare lists of outgoing links between
versions of a single tiddler
* Use new extractLinks method in backlinks indexer
...rather than copy-pasting the implementation
* Remove ES6-isms
TiddlyWiki needs to work with browsers that only support ES5
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