WIP: Add Redcarpet::Markdown#initialize_copy

I tried to implement `Redcarpet::markdown#initialize` to make
`markdown.dup` and `markdown.clone` work, but I couldn't.
(`markdown = Redcarpet::Markdown.new(Redcarpet::Render::HTML)`)

Somehow `Data_Get_Struct(self, struct sd_markdown, sdm_self);`
at ext/redcarpet/rc_markdown.c:143 assign null into `sdm_self`
when `markdown.dup` is called.
`markdown.send(:initialize_copy, markdown2)` works though.

Could you give advices to me?