A tiny guard in the grammar makes parsing faster
Make everything work from within Vimwiki

- via :Vimwiki2HTML and :VimwikiAll2HTML
- and parsing Wikilinks works

I think its better if the matching of typefaced text and links is not
done by the grammar, but rather “manually”, because matching something
like “every text, but not Wikilinks and other links and bold text
and italic text and bold italic text ...“ is very complicated and slow
with PEG grammar, which knows only greedy matches.