Optional vertical rhythm

[bramsmulders]

Thinking of the following idea, in addition to: https://github.com/Blocss/blocss/commit/61fd36ef9a14db082b4490122555bf338bb3d298

.line--flat {
    line-height: 1;
}
.line--small {
    line-height: 1.25;
}
.line--normalis {
    line-height: $line-height-ratio;
}
.line--large {
    line-height: 1.75;
}