Add script to generate csv from *.md files

• common.py: App dataclass + validation (e.g. checks possible categories)
• md_parser.py: parses *.md to App object. This could be used to validate MRs as well (i.e. parse the changed *.md and check that the generated App object is valid).
• md_to_csv.py: generates csv from directory with *.md files

NOTE: The output is not exactly the same as the current csv (but I think that shouldn't matter?).

fixes #36 (closed)