Add color tags for colors in LESS mixin arguments

A reference to a mixin with a color as an argument will now show a color tag.
(especially useful for files like [this](https://github.com/SLaks/SLaks.Blog/blob/gh-pages/css/_colors.less))

IntelliSense still doesn't show a color picker :frowning:

I think the problem is that `Microsoft.Less.Editor.Intellisense.MixinReferenceListCompletionContextProvider` is suppressing all IntelliSense inside mixin reference arguments.

Can you fix that?
It would be very cool if you could figure out what IntelliSense items to show based on where the parameter is used in the mixin.