improve: Ignore mention in Markdown code block

When publish a topic following:

    @someone you can `git clone git@github.com:jysperm/RootPanel.git`

It will be rendered to:

    @someone you can git clone git[@github](/user/github).com:jysperm/RootPanel.git

This commit will resolve it.

Known Issues: this commit only handle mentions in `code`, but not deal mentions in `pre` block.