New Features

* Add `background-clip` and `clip` to `smacss` preset sort order
* Add `LengthVariable` linter for reporting when variables should be used
  instead of literal lengths

Changes

* Increase minimum required `sass` gem version to 3.4.20+
* Modify `Shorthand` linter to report lint if a shorthand of a length not
  specified in the `allowed_shorthands` option is used
* Ignore SCSS functions in `NameFormat` linter
* Update default formatter to output column number in addition to line number
* Wrap `@`-declaratives in backticks in lint messages

Bug Fixes

* Fix `PropertySortOrder` `separate_groups` option to work for preset sort
  orders
* Fix `TrailingZero` to report correct lint messages for values with multiple
  trailing zeros