TargetTimeAdjuster will adjust a list of Features until it is within an optional
margin of a target total duration.

Helper functions:

 - _determine_margin() :: figure out the max and min cutoff times, considering
   margin and margin strategy (percent / absolute)
 - _features_total_time() :: basic sum of list of Features' durations

TODO: rename to TargetDurationAdjuster ? rename 'strategy' ??