EHN: Add index parameter to to_json
- [ ] closes #17394
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Examples:
```
In [1]: df = pd.DataFrame([[1, 2, 3], [4, 5, 6]], columns=['a', 'b', 'c'])

In [2]: df.to_json(orient='split', index=True)
Out[2]: 
{"columns":["a","b","c"],"index":[0,1],"data":[[1,2,3],[4,5,6]]}

In [3]: df.to_json(orient='split', index=False)
Out[3]:
{"columns":["a","b","c"],"data":[[1,2,3],[4,5,6]]}

In [4]: df.to_json(orient='table', index=True)
Out[4]:
{"schema": {"fields":[{"name":"index","type":"integer"},{"name":"a","type":"integer"},{"name":"b","type":"integer"},{"name":"c","type":"integer"}],"primaryKey":["index"],"pandas_version":"0.20.0"}, "data": [{"index":0,"a":1,"b":2,"c":3},{"index":1,"a":4,"b":5,"c":6}]}

In [5]: df.to_json(orient='table', index=False)
Out[5]:
{"schema": {"fields":[{"name":"a","type":"integer"},{"name":"b","type":"integer"},{"name":"c","type":"integer"}],"pandas_version":"0.20.0"}, "data": [{"a":1,"b":2,"c":3},{"a":4,"b":5,"c":6}]}
```