Writing the StructuredText#as_text method

Much needed to simplify representing single-block StructuredText field like titles, and framing them with the proper `<hn>` tag for instance. Today looks like this:

```ruby
document['fieldname'].blocks[0].text   # hopefully, there's really only one block!
```

Will be much better when it looks like this:
```ruby
document['fieldname'].as_text
```

Let me know if you think it should be in the starter kit, but I put it there because it's pretty similar to (and pretty much as useful as) `as_html`.