Nested lists

Hello!
I was trying out a few things with the markdown parser, and noticed that it doesn't seem to handle nested lists. I don't think nested lists are actually in the markdown "spec," but it seems that implementations often support them. Thanks for your work on this project!

``` haxe
var myMarkdown = `
- item 1
- item 2
- item 3
- item 4
- item 5
`;
```

Results in this markup:

``` html

• item 1


• item 2
  - item 3
  - item 4
  


• item 5


• item 6