docs(btn): remove unneccessary stuff from bemsjon demo
feat(image): add block "image"
feat(hr): add block "hr"
fix(image): add missing deps file
feat(link): add block "link"
feat(page): add block "page"
feat(scaffolding): add core block "scaffolding"
feat(sr-only): add block "sr-only"
refactor(mixins): get rid off redundant mixins
refactor(glyph): simplify API
feat(heading): add "heading" block
feat(clearfix): add "clearfix" block
refactor(btn): move require blocks to "require" section