pylint fixes
Dont display useless spec url when --prebuilt
check_base: attachments without header, change default sort key.
check_base, checks: support tests returning NA + attachments.
plugins: use default attachment sort key, not hard-coded 10.
Provide better feedback on active plugins and flags.
Use new mechanism for all f-r state output at end of report.
Support display and modification of active plugins

Adds two new options --display-plugins and --plugins to display
and modify what plugins to run, the decision by default made
by is_applicable().