[Feature Request] Fix unexpected semantics of el-color-picker

### Existing Component
Yes

### Component Name
el-color-picker

### Description
I have three issues with the current `el-color-picker` semantics. I do not know if some of these can be considered a bug. The issues are:

(1) I would expect the v-model value to be updated "live". Now the value is only updated after pressing the `Clear` or `OK` button. In other components, like `el-input` or `el-slider`, the v-model values are also updates directly. Now you have to use the `active-change` events to keep track of the current value.

(2) The format of the color in the `active-change` event differs from the default one. When not specifying anything, the color is passed as rgb(r, g, b) instead of #rrggbb. This is a bug I think.

(3) When closing the color picker popup by clicking somewhere else, no `change` event is generated. In fact, at this moment there is no way to see whether the color picker was closed.