Add color tags for colors in LESS parameters & arguments

Color tags are now shown in mixin references, function calls, and default values in mixin definitions.
(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:  (except for default values in mixin definitions)

I think the problem is that `Microsoft.Less.Editor.Intellisense.MixinReferenceListCompletionContextProvider` is suppressing all IntelliSense inside mixin reference arguments.

Can you fix that? (in Microsoft.VisualStudio.Web.Extensions.dll)
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.
This would be especially useful for mixins that simply add vendor prefixes.