add support for toggling all subsequent unit translations

[sporeventexplosion]

Make it possible to toggle px unit translations on and off for multiple declarations:

.style-a {
  margin: 16px;
}
/* postcss-px2units-disable */
.style-b {
  padding: 48px;
}
/* postcss-px2units-enable */
.style-c {
  font-size: 24px;
}

is transformed to

.style-a {
  margin: 16rpx;
}
.style-b {
  padding: 48px;
}
.style-c {
  font-size: 24rpx;
}

In some environments (such as many of the proprietary "miniprogram" runtimes), it may be desirable to style parts of a page with dimensions proportional to the page width and other parts with absolute "device pixel" dimensions. This can be done by using px and rpx units together. However, with the current version of postcss-px2units, a large number of /* no */ comments is needed if the developer wants to do this on a page with postcss-px2units enabled. This change makes it more concise to use the two units together.