it only support 10 levels

[mnlin0905]

When I try to load more than 10 levels of structure, like

* 1
  * 2
    * 3
      * 4
        * 5
          * 6
            * 7
              * 8
                * 9
                  * 10
                    * 11

It can't draw the required graph correctly

Without "11"

So, what should I do?

[mnlin0905]

@dundalek markmap/markmap#47
Here you have a hard look: are these two problems similar?