stops image/file drop onto editor markdown causing havoc
stops image/file drop onto editor markdown causing havoc

 closes #391

 - added dragDrop falsey value to codemirror instance

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