Create manager for command line defaults

Factor out responsiblity for determining correct default value for a
command line option to a manager class.
Set option's default value from config file

Add support for using config files to set the default value for command
line options.
Add config file support for command extensions

The command line extension decorators now support loading command line
option defaults from config files.
Fix missing keyword argument for config section

The create_parser() function did not accept a keyword argument for
setting the config file section used to read option values from.