Easier inclusion/setup

[koppen]

Starting a new project, it can get tedious having to include the Substrate Core mixins everywhere, ie doing stuff like

@import '~substrate-css/core/form'

.form

input[type=password],
input[type=text],
  +substrate-form-input

label
  +substrate-form-label

textarea
  +substrate-form-textarea

It would be nice if Substrate included an opt-in file for this I could import, ie so I'd only have to do something like:

@import '~substrate-css/core/form/elements'