Using @variables in colorscheme writing

Using of @variables 'a la css' for writing colorschemes would increase consistency and quality of newly written colorschemes and allow easier editing of existing ones.

<key>background</key>

<string>@darkgray</string>

<key>caret</key>

<string>@fancyyellow</string>

<key>foreground</key>

<string>@almostwhite</string>