Modify command

Adds the modify command as suggested in #60 (closed). For now, the append-mode is not implemented because this will require the parsers and entries to be refactored somewhat which is going to happen in #49 (closed). Once that is done, the append mode should be more straight forward to implement.

Thus, be mindful of how you use this command as it will always replace the contents of the specified field with the new value. Here, the new git-integration may come in especially handy.

• Modify Command
  • CLI implementation
  • TUI integration
  • documentation
  • unittest