ENH: dataframe to markdown doesn't have option to hide index
Suppose I have the following `pd.DataFrame`:

```
df = pd.DataFrame({'a': [1,2,3], 'b': [4,5,6]})
```

If I want to print it without the index, I could do:

To string:
```
df.to_string(index=False)
```

To html:
```
df.to_html(index=False)
```

To json:
```
df.to_json(index=False, orient='table')
```

But it fails in `to_markdown`:

```
df.to_markdown(index=False)

TypeError: tabulate() got an unexpected keyword argument 'index'
``` 

Shouldn't `to_markdown` method have the option to hide the index too?