Fix some issues with the list extension

This pull request improves the rendering of subsequent lists. It solves some issues highlighted in #30.

Rendering fixes

* Unordered and ordered lists are now always separated.
```
- item1
- item2
- item3

1. item1
2. item2
3. item3
```
* An unordered list must retain the same list marker throughout the list, otherwise it will be split
```
- item1
- item2
- item3

* item1
* item2
* item3
```

Note: #30 mentions an issue when parsing this code:
```
- item1
- item2
- item3

test
1. item1
2. item2
3. item3

* item1
* item2
* item3
``` 
This PR does not change the rendering of this code, since it is not wrong. To make this example work as expected use the following markdown code:
```
- item1
- item2
- item3

test

1. item1
2. item2
3. item3

* item1
* item2
* item3
```