* FIX#5183 : zimwiki : remove automatic colon prefix before internal images
 should be converted to {{foo.png}} (relative path)
Therefore the ':' prefix is useless and must be removed.
I never used the zimwiki, but i submitted the similar dokuwiki fix.
1. The zimwiki syntax is inspired by dokuwiki
2. The zimwiki documentation does not mention the colon character for images
3. The pandoc zimwiki writer seems to be a copy-paste for the dokuwiki writer
If the PR #5184 is applied, I think this one should be applied too.
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.
