Dude this feature would be awesome. It would be absolutely sick if it could be implemented with keyboard shortcuts for example cmd + opt + 1 for heading 1 for like titles. Then cmd + opt + 2 for heading 2 and all the way up to 6.
Also seriously I don't get those that are saying, "if you want this feature look elsewhere." Seriously this is a very robust and convenient note taking program. Syncs with multiple devices, has web plugins, nice sharing features, decent organization. Going to a different solution for note taking would be very inconvenient because not many have all these features Evernote has. Adding markdown capability would not be outside of what Evernote is meant to do. It would go hand in hand with what it's meant for.