Refactor CSS unit transforming

[malash]

This PR introduced a new option css.unit in goji.config.js to enable CSS unit transforming.

css.unit

• Type: undefined | 'keep' | 'to-px' | 'to-rpx'

• Default: 'to-rpx'

There is a built-in PostCSS plugin called postcss-transform-unit that can be used to transform CSS
units in all files. In some cases, you may want to convert the usage of px to rpx or vice versa.
You can use this option.

• keep: Do not transform any CSS unit.

• to-px: Transform rpx to px. 2rpx equals to 1px.

• to-rpx: Transform px to rpx. 1px equals to 2rpx.

The default will be changed to keep in the future.