Intellisense: Color variables for LESS and SCSS (#178)

Intellisense: Color variables for LESS and SCSS (Fixes https://github.com/madskristensen/WebEssentials2013/issues/178).

It will also take these into consideration:

(LESS)
```less
@nice-blue: #5B83AD;
@light-blue: @nice-blue + #111;
```

(or SCSS)

````
$nice-blue: #5B83AD;
$light-blue: $nice-blue / #111;
```

There is one tricky case which [this](https://github.com/am11/WebEssentials2013/blob/ee0b77bf7054858999d5a968500790b686e6b0ff/EditorExtensions/Shared/Completion/ColorSwatchIntellisenseProvider.cs#L41#L43) doesn't resolve, that is; when variable's value is color math expression with other variables and implicitly has color value (`$foo: $bar + $jar;`, where `bar` and `jar` has color value).

@spadapet, for the latter scenario, can we get parsed variable from `ScssVariableDeclaration` or `LessVariableDeclaration` objects' value?