Improve add_duration

[KES777]

This is related to #108

When adding month this is long to write:

 $date->add( months => 1, end_of_month => 'limit' )

Would be cool if end_of_month parameter could be specified for object itself $date. So for this date math uses limited scheme

 $date->end_of_month( 'limit' )->add( months => 1 )

[KES777]

Actually I suppose some flag already implemented and stored at DateTime object, this is how preserve works

[autarch]

The flag is stored in the DateTime::Duration object, not DateTime. I'm not too keen on adding the flag to DateTime objects. It's just yet more state in there that can make things confusing.