Feature Request: Multiple date formats

### I'm Submitting a ...

```
[ ] Bug report
[X] Feature request
[ ] Support request
```

Since it is possible in moment.js multiple date formats to use for parsing, it would be very nice if react-datetime also had this feature. At the moment, if an array of date formats is set to dateFormat property, then the react-datetime component crashes.

Inspect the react-datetime component's dateFormat handling and its Moment.js parsing behavior. Reproduce the crash with an array of formats, then review the existing component behavior for a single format. Done means array-valued dateFormat inputs are accepted without crashing while existing single-format behavior remains intact.

Written by the indexing model from the issue text.