Add color tags for colors in LESS mixin arguments

IntelliSense still doesn't show a color picker :frowning:
Add color tags for default values in LESS mixins

(for declarations)

The color picker already works here, although clicking the adornment to
open it doesn't quite work.
Add color tags for LESS function arguments

The color popup does not work here
Create CssItemAggregator to avoid duplicate crawl

GetColors() now directly finds all color nodes regardless of parentage.
Use CssItemAggregator in CSS Navigate To

Also change the parallel loop to not re-iterate the files collection
every time
Don't show color tags in selectors

Only look for color name identifiers in values.
It might be faster to change this to a nested crawl; I'm not sure.
Merge pull request #16 from SLaks/mixin-reference-color

Add color tags for colors in LESS parameters & arguments