[Feature Request] Add "before-close" attribute to <el-popover>, just like <el-dialog>

### Existing Component
是

### Component Name
el-popover

### Description
` also need ":before-close" attribute, just like `

Then the source code can be written like below

```javascript

...
...

export default {
methods: {
handleClose(done) {
this.$confirm('confirm close?')
.then(_ => {
done();
})
.catch(_ => {});
}
}
};

```