Fix GFM blocks within lists

A couple of fixes going on here:

1. GFM-style code blocks were not rendered correctly in lists. For example:

```
* Your directory tree should be similar to this: 
   \`\`\`
    + the-openvj-project/       (any name)
      + openvj/                 (git://openvj)
        - openvj-package.box
        + .git/
        + doc/
        + src/
        - …
      + openvj-git-service/     (git://openvj-git-service)
        + .git/
        - …
      + openvj-bg-service/      (git://openvj-bg-service)
        + .git/
        - …
      + openvj-data/
        + git/
    ```
```

2. Tagential to the above, any number of spaces before a GFM style block should be permissible:

```
 \`\`\`
+ the-openvj-project/       (any name)
  + openvj/                 (git://openvj)
    - openvj-package.box
    + .git/
    + doc/
    + src/
    - …
  + openvj-git-service/     (git://openvj-git-service)
    + .git/
    - …
  + openvj-bg-service/      (git://openvj-bg-service)
    + .git/
    - …
  + openvj-data/
    + git/
 \`\`\`
```