Code fences inside of lists exits list

[albertyw]

Possibly related to #275 or #263

Adding a code block inside of a list results in the parser immediately exiting the list <li> and <ul> / <ol> for the code block and anything below it. Note that this is with "fenced-code-blocks" enabled.

For this markdown:

1. asdf
    ```
    asdf
    ```
    asdf
1. qwer

---

python-markdown2 produces:

1. asdf

asdf

asdf

1. qwer

---

Github markdown syntax produces:

1. asdf

   asdf
   

   asdf
2. qwer

[momja]

@trentm this issue can be resolved