Combine breakpoint-prefixes

[nelhop]

When responsive breakpoints only change in a small viewport (i.e. sm) all above breakpoints have to be added, which leads to a long and hard to read class-string.

It would be handy to combine multiple breakpoint-prefixes at once, to avoid duplicate code. For example like md:lg:u-w-class

Instead of writing

sm:u-w-10/12 md:u-w-8/12 lg:u-w-8/12

it would be handy to write

sm:u-w-10/12 md:lg:u-w-8/12