Chunk param

Closes #244

Allows to move to a specific chunk at startup by defining a route param.

If we have the following in one of our configuration files:
```
{
  "main" : {
    "chunkParam" : "chunk"
  }
}
```
we can add a param `?chunk=SENTENCE_ID` to our route. Right now the param has the an id of a sentence. The application will start at this sentence, if a sentence with such an id exists. If not, an alert is given and the first sentence of a document is loaded.

This param will be revalued once navigation gets more complex. It could be used to load several sentences at once for example, but this is currently out of scope.