Add a 'hide duplicates' option

I know #14 was about adding the ability to sort by color in order to find duplicates, which is very handy, but I have a project that has duplicates _on purpose_ (i.e., setting a less variable like `@error-color` to another less variable like `@red`). It'd be nice to be able to toggle duplicates on and off, to get an overview of the unique colors in the project.

What do you think?

This is a very useful package, thanks!