Rowspan feature

- Have you already searched for similar issues? yes

- I am looking for Rowspan feature in react-data-grid library. Is there a way to handle it?
Rowspan using Groupby.
- Thanks!

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### Which version of React JS are you using?

✅ Officially supported ✅
- [x] v15.4.x

⚠️ Not officially supported, expect warnings ⚠️
- [ ] v15.5.x
- [ ] v15.6.x

☣️ Not officially supported, expect warnings and errors ☣️
- [ ] v16.x.x

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### Which browser are you using?

✅ Officially supported ✅
- [x] IE 9 / IE 10 / IE 11
- [x] Edge
- [x] Chrome

⚠️ Not officially supported, but "should work" ⚠️
- [ ] Firefox
- [ ] Safari

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### I'm submitting a ...

- [ ] 🐛 Bug Report
- [x] 💡 Feature Request

> 👋 Need general support? Not sure about how to use React itself, or how to get started with the Grid?
> Please do not submit support request here. Instead see

> https://github.com/adazzle/react-data-grid/blob/master/CONTRIBUTING.md

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### Issue Details

```
Please include:
- What the current behavior is
Rowspan feature is not supporting
- What the desired behvaior is
Support Rowspan feature
- (If Bug) Steps to reproduce the issue
- (If Feature) The motivation / use case for the feature
GroupBy rows if other rows have same values.
We especially love screenshots / videos of problems, and remember
The Best Issue Is A Pull Request™
```