exclude FencedChunk from link conversion (fixes None.get)

related tristanjuricek/knockoff#36

## steps

create a page with the following content:

    ## foo

    ```scala
    val turtlePosition = Lens.lensu[Turtle, Point] (
      (a, value) => a.copy(position = value),
      _.position)
    val pointX = Lens.lensu[Point, Double] (
      (a, value) => a.copy(x = value),
      _.x)
    ```

## problem

```
HTTP ERROR: 500

Problem accessing /foo.html. Reason:

    None.get
```

## notes

This was likely introduced when I upgraded knockoff to 0.8.1.
There's a conversion process which tries to resolve link formats, and by default only `IndentedChunk` is excluded from it. I just added `FencedChunk` to the exclusion, and that seemed to solve the issue for this case.

If someone had malformed link in non-code text, she could still hit `None.get`.