Option to put props on single line

Hi, cool library. It turned out to be exactly what I was looking for today.
Anyway, I noticed that the JSX string adds line breaks between props when the component has children. This leads to somewhat awkward looking strings.

For example,
```javascript
jsxToString(Another thing, {
shortBooleanSyntax: true
});
/*
Outputs:

Another thing

*/
```

My proposal is to add an option to put props on one line. For example,
```javascript
jsxToString(Another thing, {
shortBooleanSyntax: true,
singleLineProps: true
});
/*
Outputs:

Another thing

*/
```

It looks like a pretty simple fix, so I may just go ahead and submit a PR.

Start at the jsxToString entry point and trace how formatting options affect props when the component has children. Add the proposed singleLineProps option, then verify that the example produces one prop line while preserving the child indentation and closing tag output.

Written by the indexing model from the issue text.