- [settings] added icons and sprites prefixes variables to settings
- [mixins] added explanation why in media query mixin `breakpoint()` used `only` declaration
- [mixins] updated `breakpoint()` to handle various maps — default `--devices`, `--custom` or even manually specified
- [settings] added predefined `$breakpoints--devices` with `retina` media query and empty `$breakpoints--custom` maps