[Bug Report] Dropdown component needs a popper class.

### Element UI version
2.15.1

### OS/Browsers version
ALL

### Vue version
2.6.12

### Steps to reproduce
https://element.eleme.io/#/en-US/component/dropdown

Need to be able to pass custom class names to dropdown component

### What is Expected?
Expect to have the ability to add custom classes, like on other components. ( popper-class )

### What is actually happening?
No options to pass extra class.