Add ```language support for markdown files

[kghbln]

trafficstars

Setup

• MediaWiki | 1.27.3 (2fd9800)06:05, 7 July 2017
• PHP | 7.0.18-0ubuntu0.16.04.1 (apache2handler)
• MySQL |  5.7.18-0ubuntu0.16.04.1
• GitHub | 1.4.1 (9b521ca) 07:39, 1 July 2017

Issue When using individual lexers like e.g. ````json` the formatting is broken. Example. In case a lexer cannot be rendered for some reason a behaviour like with the "pre" tag will be nice.