SCSS to generate display helper classes

[chimericdream]

Note: Once Bootstrap 4.0 is finalized, this feature should follow the same convention they use for their responsive display utilities.

In case the branch is deleted after the release, the file in question is bootstrap/scss/utilities/_display.scss
v4-dev branch of _utilities.scss

Reference
twbs/bootstrap#18410 (comment)
twbs/bootstrap#20004

@if $enable-flex {
    .d-flex {display: flex;}
}
.d-block {display: block;}
.d-inline {display: inline;}
.d-inline-block {display: inline-block;}

@each $breakpoint in map-keys($grid-breakpoints) {
    @include media-breakpoint-up($breakpoint) {
        @if $enable-flex {
            .d-flex-#{$breakpoint} {display: flex;}
        }
        .d-block-#{$breakpoint} {display: block;}
        .d-inline-#{$breakpoint} {display: inline;}
        .d-inline-block-#{$breakpoint} {display: inline-block;}
    }
}