Better lists

Improved the support for ordered and unordered list translation and added support for nested lists.

The following are all generated from the expected HTML:

``` markdown
Simple ordered list

1. ordered-item1
2. ordered-item2
3. ordered-item3

Simple unordered list

* unordered-item1
* unordered-item2
* unordered-item3

Nested ordered list

1. ordered-item1
2. ordered-item2
  1. ordered-sub-item1
  2. ordered-sub-item2
  3. ordered-sub-item3
3. ordered-item3

Nested unordered list

* unordered-item1
* unordered-item2
  * unordered-sub-item1
  * unordered-sub-item2
  * unordered-sub-item3
* unordered-item3

Mixed nested list 1

* unordered-item1
* unordered-item2
  1. ordered-sub-item1
  2. ordered-sub-item2
  3. ordered-sub-item3
* unordered-item3

Mixed nested list 2

1. ordered-item1
2. ordered-item2
  * unordered-sub-item1
  * unordered-sub-item2
  * unordered-sub-item3
3. ordered-item3
```

A new test fixture has been added for this functionality.