[Feature Request] Add router-link support to el-button component

### Existing Component
Yes

### Component Name
el-button

### Description
I'd like to be able to specify a route using vue's router-link, like the following:

```html
Home
Page2
```

This currently isn't supported, so I have to wrap all `el-button` components in a `router-link` component. It's pretty tedious, coming from using vue-material, which supports router-link buttons.