* FIX#5183 : remove automatic ':' prefix before internal image links
`![](foo.png)` should be converted to `{{foo.png}}` (relative path)
`![](/foo.png]` should be converted to `{{/foo.png}}` (absolute path)
Therefore the ':' prefix is useless and must be removed.
It blocks users from making relative image links.
Update tests for DokuWiki Writer : external images
This is now the default for pandoc's Markdown.
It allows whitespace between the two parts of a
reference link: e.g.
[a] [b]
[b]: url
This is now forbidden by default.
Closes#2602.