Display a leading zero if the number is only one digit long (00-59).
Display minutes in one digit otherwise (mm:ss.ms format, 0-59).

- Attribute 'tag_proc_time' defined outside __init__
- Catch the correct exception (OverflowError)
- Display '?' if no CPU times are available

Support remains there, but no system-wide configuration file will be
provided anymore.

Default configuration settings are initialized by code which may be
overridden by a configuration file.

Convert former getcapacitypercent method to property.

Docker plugin now use the stats method as default grabber. The network stats do not work as expected...

Introduce the new sort_key method. Replace the old multitude ones,
thanks to properties too.

A setter method always takes a parameter and does not return a value.

Rename 'set_stats_snmp' method to 'get_stats_snmp'.

Don't redefine built-ins. Unless it's wanted.

Empty sets evaluate to False.

- Remove unused import
- Fix double import
- Remove unnecessary pass statement
- Fix bad indentation
- Fix pointless statement

Use None as a default value.
See https://stackoverflow.com/questions/101268/hidden-features-of-python#113198
for more information.