Add prop: exact-active-class

### What problem does this feature solve?
I want to use both `active-class` and `(exact)active-class` at the same time.


### What does the proposed API look like?
```
<router-link
  :to="location"
  active-class="menu__item--active"
  exact-active-class="menu__item--exact-active">
```
It can replace the original prop `exact`.