Fix modified date handling; add env.var support for version.

[dalito]

• Support for two environment variables was added. Both changes enable achieving consistent version/tag info in the gh-based vocabulary management (related Finalize deployment to gh-pages voc4cat-template#11).
  • VOC4CAT_MODIFIED - Can be used to supply a modified date. If present it will be used with highest preference.
  • VOC4CAT_VERSION - version string; it must start with "v". If present this version has highest precedence.
• In the concept scheme sheet in xlsx, version strings must start with "v", too. This prevents that Excel auto-format modifies the version string by falsely interpreting the version as date or number.
• The version string and the modified date are now optional. Tracking version or modified date in the code that is under git control does not make much sense. This info should be only added in the release step.

This PR also fixes the link to the RDF-file in pylode-docs.

Closes #147