Rename 'cumulative' to 'use_stack' in Stagecraft stubs.

We've been using an option called 'cumulative' to indicate whether timeseries
graphs should use line or stacked graphs. This is confusing, especially now
some of our graphs do show genuinely cumulative data.

Rename to 'use_stack' throughout.

Pivotal story: https://www.pivotaltracker.com/s/projects/911874/stories/63460908
Merge pull request #229 from alphagov/clarify-naming

Rename 'cumulative' to 'use_stack' in Stagecraft stubs.