[bug] unexcepted behavior and bug with codeblock

[yehya]

1. add new codeblock in preview mode, then switch editor to markdown mode will cause error

2. unexcepted behavior in html mode, when add new codeblock in preview mode repeatedly.
   (when click codeblock button twice, html tag should be switched create tag and remove tag, not create tags nested)

Ps: Enable to edit markdown in preview mode is awesome! I really love this editor!