Make it possible to provide a renderable as a label to a progress bar

Other components do it (`<TabbedArea>` for the tab label comes to mind), I don't see why we wouldn't want to generalize a bit here, if anyone wants to add a child to an already nested progress bar or if someone wants better control over how the percentage is displayed (currently the value is ceiled).
I wonder if we should still respect `this.props.srOnly` prop here, we could reuse the `renderScreenReaderOnlyLabel` function but make it so you pass in the label not the percentage.
Line 88 would probably need to change to something like:
```
this.renderScreenReaderOnlyLabel(this.renderLabel(percentage)) : this.renderLabel(percentage);
```