If you want something dynamic, that can actually give you a list of function calls, or variables you've made, etc, etc, you'll want TernJS. This StackOverflow answer goes pretty in-depth on how to use it with Atom.
Otherwise, if you just want auto-completion for things like ng-app or $scope, or other common angular things, you could try this package, though I haven't used this myself, so I don't know how well this package works.