Multi-brand support

[mgrsskls]

• [ ] One config file per brand, inheriting from main config file (e.g. .miyagi.js, .miyagi.brand1.js, .miyagi.brand2.js). This allows different theming of miyagi itself as well as different CSS files etc for the actual project.
• [ ] A brand switch in the miyagi UI which toggles the brand on-the-fly
• [ ] miyagi build would create a build including all brands, miyagi build --brand <brand> would create a build without the switch

The term brand is just temporarily, should probably be changed.