ENH Enable folding when calling `model_card.add`

Related to: #315 

When calling `model_card.add` you aren't currently able to set the `folded` property of each section without selecting each section separately and manually setting the folded property. Adding the ability to do with in `model_card.add` will help clean that up. 

Example: 

```python
model_card.add(**{"foo/bar": "Foo/Bar", "foo/baz": "Foo/Baz"}, folded=True)
``` 