Issue #1977
Most markdown processors support the [shortcut format] for reference links.
Pandoc's markdown reader parsed this shortcuts unoptionally.
Pandoc's markdown writer (with --reference-links option) never shortcutted links.
This commit adds an extension `shortcut_reference_links`. The extension is
enabled by default for those markdown flavors that support reading shortcut
reference links, namely:
- pandoc
- strict pandoc
- github flavoured
- PHPmarkdown
If extension is enabled, reader parses the shortcuts in the same way as
it preveously did. Otherwise it would parse them as normal text.
If extension is enabled, writer outputs shortcut reference links unless
doing so would cause problems (see test cases in `tests/Tests/Writers/Markdown.hs`).
