Should not purify 'bg-*' and 'text-*' classes

[sebastienroul]

Using variables in some ways to set the color (or bg-color) is not reconized by purifycss even if they try to thing the more crazy things [https://github.com/purifycss/purifycss#detecting-the-use-of-button-active-1]
So, always adding colors in the final packaging could be great for dev more crazy than puryfyccss's team :o) or people using color as functional feature....

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