Line numbers code Markdown

[Luvideria]

referring issue #562

Line numbers were available in the SyntaxHighlighter.
Syntax highlighting is now handled by Markdown code.
As per #562 (comment) this issue is for this specific feature to be reintroduced.

[wbrgss]

We could put a show_line_numbers flag in highlight_config that if set would call the init function for this plugin when the Markdown component is mounted.