Merge branch 'dev' of https://github.com/youzan/vant into dev

[new feature] Card: add bottom slot

[Doc] update README.md

�[bugfix] Pagination: should not emit change event when total-items c…

…hanged

[Doc] add doc version tip

[build] update release.sh to support publish beta package

[bugfix] Utils: <is-src> data URLs should be prefixed with the data:i…

…mage (youzan#3308)

[improvement] check event.cancelable before preventDefault

[bugfix] Step: incorrect active step when insert step asynchronously

Merge branch 'dev'