[Feature Request] Inconsistent type naming for Alert and Tag components

### Existing Component
Yes

### Component Name
Alert, Tag

### Description
Hi, Element UI provides multiple `type` value options for both **Tag** and **Alert** components, i.e. `success`, `info`, `warning`. However, the `type` value for **error** context is inconsistent. For **Tag** it's called `danger` and for **Alert** it's called `error`. This causes a little trouble when creating a filter function because the filter function will need to return different values for the same status to represent **error** context. Could we consider provide a `type` alias feature so that those two components will be compatible with receiving both `error` and `danger` as prop `type` value?