支持多个单位配置，比如 rpx

[shepherdwind]

使用方式

    const input = 'h1 { margin: 0 0 20rpx 20rpx; font-size: 32px; line-height: 1.2; letter-spacing: 1rpx; }';
    const output = 'h1 { margin: 0 0 0.2rem 0.2rem; font-size: 0.64rem; line-height: 1.2; letter-spacing: 0.01rem; }';
    const processed = postcss(pxtorem({
      rootValue: { px: 50, rpx: 100 },
    })).process(input).css;

    expect(processed).toBe(output);

[coveralls]

Coverage increased (+0.2%) to 98.462% when pulling 67e1d50 on shepherdwind:rpx-support into c4cf164 on ant-tool:master.

[soda-x]

0.7.0