Make it possible to extract a list of texts

This equips `JResult` with a `getTextList()` method that returns a list of all `<p>` elements. The result allows us to take control over the text output format in cases where a big chunk of plain text just don't cut it.

Example usage:
```java
for(String text : extractor.extractContent("<html><body><p>Hello</p><p>World</p></body></html>")) {
  System.out.println(applyFancyParagraphFormatting(text));
}
```

Closes #30