Use new `.breadcrumb-item` class instead of child selectors and `li` tag selectors,
so as to no longer require the usage of `<ol>`-based markup.
Rename variables to follow naming conventions:
* $breadcrumb-padding-vertical => $breadcrumb-padding-y
* $breadcrumb-padding-horizontal => $breadcrumb-padding-x
Introduce new variable: $breadcrumb-item-padding
[skip sauce]