feat: fit #40

Is your feature request related to a problem? Please describe.
when you cannot both set width or height, you would need this CSS ability: object-fit, otherwise img would be distort in Safari

Describe the solution you'd like
like element: https://github.com/FEMessage/element/blob/dev/packages/image/src/main.vue#L16

Describe alternatives you've considered
just using custom css if don't care about IE11 compatibility