Support markdown definition list syntax

When writing documents, like README.md e.g., the author might have to create a list of definitions. In HTML this can be done with <dd> and <dt> and several markdown dialects–most importantly pandoc–have a syntax for definition lists as well. Cf. pandoc example #19.

Term 1

:   Definition 1

Term 2 with *inline markup*

:   Definition 2

        { some code, part of Definition 2 }

    Third paragraph of definition 2.

Unfortunately when these occur in the README.md, GitLab will not properly translate them into HTML for the user to read.