Handles "down" responsive behavior properly

[kettanaito]

GitHub

• Fixes Prop with explicit "down" behavior overrides implicit "up" behavior of preceding prop #119

Roadmap

• Add missing integration test for applyStyles() function
• Rewrite applyStyles() to use a single .reduce()
• Create prop alias groups of pristine props before generating CSS strings
• Transform prop alias group's records to close those which go down and have preceding record
• Add missing E2E tests for multiple "down" responsive props

[kettanaito]

Should cherry pick the missing E2E tests related to the issue from #198

[coveralls]

Coverage increased (+0.3%) to 87.083% when pulling 0a585ff on 119-responsive-down-issue into a76a2bf on master.

[kettanaito]

I'd like to learn more about a user's expectations when dealing with different combinations of responsive props (especially including contradicting behaviors like up/down pairs).