Manipulate the behavior of the selected text

Hi Rahman,

I have just made some modifications about the specific shortcuts manipulating the selected text such as bold, italic and code ('`' '`') selections.

We can change the behavior of the selected text handily using these changes. Here is the gif showing what the PR can do with the selected content:
  
![keyframe](https://cloud.githubusercontent.com/assets/1964927/5154096/f152a810-7250-11e4-974e-e4e390ae248f.gif)

So as usual, what do you think about it? :)