add onDrawImage

[RenShine]

If somebody want select or upload image to server before drawImage, He can set options onDrawImage.
example:
var simplemde = new SimpleMDE({
element: document.getElementById("Id"),
onDrawImage: function() {
//do select and uploadImage
simplemde.drawImage("imageUrl")
}
});

[sweetXiaoyan]

onDrawImage 方法没有执行，不知道如何使用 onDrawImage