As @sylumer mentioned, there are indent / outdent example actions which ship with Drafts. If you don’t have them, they can be re-installed from the action directory (Indent, Outdent).
By default these are assigned the keyboard shortcuts ⌘-[ and ⌘-]. These are the common assignments for indent / outdent in most plain text editors on Mac.
I’m not crazy about overriding the functionality of literal keys that type characters, because it disables your ability to type that character in the editor.