Specs for per-feature-unique percentages of actors
Offsetting mod key based on feature name

- this ensures that when a feature is enabled for a percentage
of actors, we don't always get back the same exact set.
- if one is attempting to consistently target the same
set of actors, using a Group to enable/disable would seem more
appropriate
Merge pull request #16 from JackCA/offset-percentage-of-actors

Offset percentage of actors