Implement rudimental prototyp for Document Publisher

[cgi1]

As a very first test, we would like to convert Document Publisher alone from CVRF 1.2 to CSAF, by establishing a general read / write workflow.

• Create first entrypoint for cvrf2csaf.py (argparse)
• Read CVRF 1.2 sample file
• Establish rudimental mapping for first XML element tree (Document Publisher)
• Save JSON to output file

[cgi1]

First code structure has been implemented by @zpevma for Document Publisher. We will push this today before tomorrows Kick Off.

[cgi1]

Closing this here, code can be seen in master repo. Basic input/output works + code structure has been presented by @zpevma and @sustefil.