fix: Transform within default variables

[NikolayMakhonin]

Transform within default variables
#79

[bart-jaskulski]

@NikolayMakhonin Already handled this in #80. Nonetheless, your regex seems less constrained. I would only suggest to write a test for that case.

[bart-jaskulski]

Anyway, I think this library might need a reliable fork, as @cuth seem not to maintain it for some time

[meduzen]

There’s one by @hemengke1997 at https://github.com/hemengke1997/postcss-pxtorem, and we’re at an early stage of using it at work, but so far it’s great.

[dimitry-blinov]

@yangmingshan @cuth Please add the ability to convert px to em inside var in the callback ignoring custom variables. Can u merge and release a new version?