Changes the behavior of Sass's @import directive to only import a file once.

This plugin changes the behavior of Sass's `@import` directive so that
if the same sass file is imported more than once, the second import
will be a no-op. This allows dependencies to behave how most people
expect them to behave and provides a considerable performance improvement
for some sass projects.

**Note**: Although this plugin is maintained by compass, it can be used
without compass in any Sass-based project.