Thanks for the request. This doesn't seem like a common need, but FWIW, I admit that from time to time I'll need it when I'm working in other tools and usually do it in my code/text editor, Atom. In the future when the app is a native Mac app, we'll get the built in benefit of OS X of the Right Click / Context Menu's Transformations option for text. In Native apps, that menu has upper/lower/capitalize.
If others would like to see this feature, please let us know (or add your Like/Heart vote above) to help us prioritize this request.