Rewriting and extending JS extractor documentation
This PR extends and adds a lot of documentation about how to write using a JavaScript extractor. Mainly describes:
- Recommends development environment using KItinerary Workbench
- Script declaration JSON and fields in it
- Additional API's that exist in JS environment
- Many examples with a lot of comments
The main driving force for this is that it was very hard for me to start adding new extractors in here, so I want to get to know better project itself, and help future people
Edited by Grzegorz Mu