Feature request: support multiple country holidays

Hello,

This is because in some county like China Japan , people also would enjoy some foreign holidays .
For example , chocolate sales would grow on Valentine's Day .
Cake and apple(yes apple) sales would have large increase on Christmas Eve .

Current behaviour
```
>>> m.add_country_holidays('CN')
>>> m.add_country_holidays('US')
WARNING:fbprophet:Changing country holidays from CN to US
```

Add a flag to determine whether appending or changing would be much better .