Disable the line numbers on a specific code block when using Markdown and Redcarpet

[tuomasj]

This is a pull request for a issue which was discussed earlier on #62.

Adds parameter handler for markdown and redcarpet renderer, currently only support line_numbers=false option.

Example:

```ruby?line_numbers=false
def my_cool_method(message)
  puts message
end

[bhollis]

Fantastic, thanks!