[Feature Request] Responsive design support

### Existing Component
No

### Component Name
Element UI

### Description
!!! Please read before closing !!!

Hello. First of all, thanks for creating this nice framework.

As I understand current position of lead element ui developers, there’s no plan to support responsive design. I would like to know the reason of this. Is it matter of workforce or ideological limitation? In theory, can responsive design be allowed if someone take care about implementation and support?

Also, is it possible to have at least partial support of responsive design? Some cases might be convenient even for desktop, giving simple example: when screen width is not enough to contain nav menu items, it collapses in way similar to bootstrap responsive menu. This may happen on desktop browser as well, when there is too many menu items.

Thanks.