Refactoring of init.php

- fetching is now done by a global function in fetch.php. This
  allows to use this functions in test/xpath.php for testing.
- besides of user configuration as explained in README.d json
  files from `mods` are now also loaded and used for fetching
  articles
- loading of json files is done only one time and not for each
  article to process
- test/xpath.php is now a script, which can be used to test a
  configuration from `mods`-directory

Added two mods for 4players and visions.de.

TODO: rewrite README.md