Add a more elegant way to insert picture in Markdown file.

[colorfulshark]

To insert a image, you just need to:

1. Add a new directory in "assets/posts" whose name is the same as
   your post file name, e.g, 2013-12-26-images. Then add a directory
   named "images" in that directory. Now you will have a directory
   "assets/posts/2013-12-26-images/images". Puts all images that you
   need into the directory "images".
2. Insert following code in markdown file:
   {% include _res/image name="image-full-name" %}. Notice that the
   "image-full-name" is the full file name of your image.
3. Rebuild.