Fix computation of continuos range length

The computation of final length when joining a continuous block has been seriously flawed. This commit should correct the computation.
Example case where the wrong computation has backfired is a list item with lazily nested paragraphs:

```
+   Lorem ipsum
dolor sit amet

consectetur
adipiscing elit
```
Merge pull request #4 from apiaryio/zdne/append-continuous-block-fix

Fix computation of continuos range length