OSX shortcut fix.
OSX shortcut fix.

closes #2573
- separate different shortcuts by OS detection in markdownEditor.js

dbalders/Ghost-Admin,airycanon/Ghost-Admin,dbalders/Ghost-Admin,airycanon/Ghost-Admin,kevinansfield/Ghost-Admin,acburdine/Ghost-Admin,kevinansfield/Ghost-Admin,acburdine/Ghost-Admin,TryGhost/Ghost-Admin,JohnONolan/Ghost-Admin,TryGhost/Ghost-Admin,JohnONolan/Ghost-Admin