Add $inherit argument to global-box-sizing and border-box-sizing

- This argument is optional and defaults to false (the original behavior).
  Meaning that this change will not break existing sites as it must be
  opted into.
- When $inherit is true, the html element alone is set to the specified
  box-sizing, and all other elements inherit from this property. This
  allows box-sizing to be changed on a single element and have it change
  on all nested elements (unless otherwise specified). This feature
  lends itself well to components that may have different box-sizing behavior.
- The changelog and relevant documentation have been updated to reflect these
  changes.