Found it this just released 5 days ago, it's precisely what I need. But I think I have permission issues. When I click the extension nothing happens. I don't see the context menu option from the screenshot as well. When I look at the extension from the extension menu, it says "Can't read and change data on this site".
By default the "Open in Freedium" menu option only shows for articles on Medium.com. Were you using on the their custom domain websites? Anyway I've just published a new version where you can add custom domains. Please give it a try!
