Add public method to retrieve current active design

What's Changed

• Add public method to retrieve active design from customer session by @robaimes in #4

Full Changelog: 1.0.0...1.1.0

Initial Release

Version 1.0 release. Supports creation of new checkout layouts, selecting default layout and layout per customer group on a store scope basis.

Initial proof of concept release

• Create new design via di.xml entry
• Use custom layouts/designs from system configuration
• Use custom layouts/designs per customer group