reopen border with rpx

[NewFuture]

Environment

• clean-css version - npm ls clean-css: 4.2.1
• node.js version - node -v: 10.x
• operating system: windows/liunx

Input CSS

.c{
  border: 2rpx solid #ddd;
}

Actual output CSS

.c{
  border: solid #ddd;
}

Expected output CSS

.c{
  border: 2rpx solid #ddd;
}

related #882

As we know that rpx is not a standard css unit，
but it's a valid unit in wxss and some processors.

How to keep it in the clean result?

1. a config to ignore the unknown unit?
2. extend units config to support user-defined unit？

[NewFuture]

as stylint dose

{
    "unit-no-unknown": [
            true,
            {
                "ignoreUnits": ["rpx"]
            }
    ],
}

it could be supported like this

{
    compatibility: {
       rpx: true,
    }
}