Update customizer

Add all the new things to customizer:
* `@headings-font-family`
* `@container-*`
* darker `@btn-default-bg`
Merge pull request #8478 from juthilo/3.0.0-customizer

Update customizer
fix broken scrollspy example
Merge branch '3.0.0-wip' of github.com:twitter/bootstrap into 3.0.0-wip
Merge branch '3.0.0-wip' into 3.0.0-sass-wip