el-date-picker应该恢复输入时多种格式的支持，而不是只支持yyyy-MM-dd
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- Dominant language
- Vue
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Description
## Element UI version
v2.14.1
## OS/Browsers version
MAC/Win
## Vue version
^2.6.10
## Steps to reproduce
目前只支持yyyy-MM-dd的固定格式输入
## What is Expected?
应该支持多种时间格式例如：yyyy/MM/dd、yyyyMMdd等，貌似在2.9.2版本以后这种支持就废弃了，为啥呢？Bug？
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