Add collecting no-prio issues. Restructure. * Move usage examples from collect_issues.py to examples/. Also add a section about this to the readme. This should make it easier to understand how it all works together. As we have two examples now, hardcode the database filenames. * Add a function to create the necessary database command from a list of columns. This makes it easier to work with tracker that have customized their priorities, * Add an example how to collect no-prio issues in a database. Note that the display part is not included. * Cleanup roundup_content_data by removing db file name suggestions and unused "reference db" command. This could be re-added for specific installations if they need it, however it probably should be separated as configuration.