Markdown: Pipe in table cells cannot be escaped

This was reported in 2012 (!) and never fixed because of a technicality: https://github.com/gitlabhq/gitlabhq/issues/1238

I've reproduced it on gitlab.com: https://gitlab.com/nkovacs/test-project/blob/master/README.md

For comparison, github does it right: https://github.com/nkovacs/markdown-test/blob/master/README.md