Changes

• cleaned-up code (see issue #12)
• fixed a bug where package thresholds were ignored (see issue #7)
• package thresholds have changed (warning/critical 5/10 to 1/5)
• new threshold parameters -W / --core-warning and -C / --core-critical for outdated core (warning/critical 1/3)
• switched from optparse to argparse
• implemented logging

Removed functionality

• parameters -l / --list-packages removed, as the behaviour is now default

• Fixed a bug where empty assignments were done (see also issue #8)
• Switched mirror lists from HTTP to HTTPS as the IPFire project changed the distribution mechanism (see also issue #10)
• Added Icinga2 configuration example (see also issue #2)
• Plugin is now executable by default
• cleaned-up code

Implemented a parameter for listing outdated packages (see issue #6)