* Fix bug in `Indentation` linter where `@at-root` directives with inline
  selectors would erroneously report incorrect indentation levels
* Fix bug in `Indentation` linter where rule nodes with selectors spread
  over multiple lines and a single inline property would incorrectly report
  a lint
* Add `ElsePlacement` linter which checks the position of `@else` directives
  with respect to the previous curly brace
* Replace `allow_extra_spaces` option on `SpaceAfterPropertyColon` linter with
  `style` which accepts multiple different property spacing styles
* Fix bug in `PlaceholderInExtend` linter which erroneously report lints for
  selectors with interpolated strings