this is something that you can actually do pretty easily. You can do it though a few different ways, so it depends what you are looking for. If you want to send this not using an email alert, you can do it entirely in a Flow with a "Send Email" element and build out your own Template and query the Users that need to be notified. Another option is to have all of your Chatter Feed Items, using Process Builder, update a field on the Case that tracks them. From there, you can run a workflow on anytime that field 'ischanged' to figure out when you need to send a notification. You can also pass in other values from the Feed Item (Feed Item = Chatter Post in API backend). This means, you could only pass out public messages, find who posted the message and grab their profile or something to create more dynamic workflow off of that... really the sky is the limit. With this solution you also get to bring in an email alert with Case Fields, since you would be firing the workflow from a Case field.