Flexible `u-object-fit()` mixin
Since the extra width: 100%; height: 100%; styles were removed from .u-object-fit-cover in #9, we've refactored u-object-fit() to work like u-position() and allow custom styles via @content.
For example:
@include u-object-fit('cover-100%', cover) {
width: 100%;
height: 100%;
}