Generate Robot Framework

I would like to use httpsnippet to convert HTTP calls to [Robot Framework](https://robotframework.org/) test cases using RF libraries like:
- [RequestsLibrary](https://github.com/MarketSquare/robotframework-requests)
- [RESTinstance](https://github.com/asyrjasalo/RESTinstance)

This feature will add huge value for API automation testers.

No files, tests, or entry points are named. Start by reviewing how existing language and library generators are organized, then determine how Robot Framework output should represent the requested RequestsLibrary and RESTinstance cases. Done means generating valid, tested Robot Framework test cases for the supported HTTP inputs.

索引モデルが issue の本文から書いたものです。