Remove special fields, bump Jinja dependency, requirements to source traceability and custom grammar documentation is public

What's Changed

• Regenerate CHANGELOG by @stanislaw in #614
• backend/reqif: export/import roundtrip: parent-child links by @stanislaw in #617
• Code climate: backend/reqif: more consistent class naming by @stanislaw in #618
• docs: remove a dead link by @stanislaw in #619
• traceability: support linking Tex files to requirements by @cbernt in #620
• Relax version spec for jinja2 by @lochsh in #623
• Bump version to 0.0.20 by @stanislaw in #624

Full Changelog: 0.0.19...0.0.20