Update Atom API according to discussion in bustle#228

ATOMS.md

@@ -2,62 +2,50 @@
 
 Atoms are effectively read-only inline cards.
 
-An atom is an object with a name and at least one of several hooks defined. For example:
+An atom is an object with a name, type and render hook:
 
 ```js
-var demoAtom = {
+// package: mda-mention-dom
+export default {
   name: 'mention',
-  display: {
-    setup(element, options, env, text, payload) {
-      element.innerHTML = `<span class="mention">${text}</span>`;
-    }
+  type: 'dom',
+  render({element, options, env, text, payload}) {
+    element.innerHTML = text;
   }
-}
+};
 ```
 
-### Available hooks
+### Teardown
 
-Like [CARDS](CARDS.md) there are several hooks an atom should define.
+Mobiledoc-kit will clear the children of a rendered atom, but there may be cases where you wish to manually teardown
+any event listeners attached to the window etc...
+
+The return value of the `render` hook is a teardown function which will be called before the atom is removed from the DOM.
 
 ```js
-var exampleAtom = {
-  name: 'example',
-  display: {
-    setup(element, options, env, text, payload) {},
-    teardown(setupReturnValue) {}
-  },
-  html: {
-    setup(buffer, options, env, text, payload) {},
-    teardown(setupReturnValue) {}
+// package: mda-mention-dom
+export default {
+  name: 'mention',
+  type: 'dom',
+  render({element, options, env, text, payload}) {
+    element.innerHTML = text;
+    let popOver = new PopOver(element, { url: payload.url });
+    return () => {
+      popOver.destroy();
+    }
   }
 };
 ```
 
-|Hook|Used by Mobiledoc Kit|Used by DOM Renderer|Used by HTML Renderer|
-|---|---|---|---|
-|`display`|✓|✓||
-|`html`|||✓|
-
-Atoms are read-only so, unlike cards, do not have an `edit` hook.
+### Arguments
 
-Each hook has a `setup` and `teardown` method. The arguments are:
+The `render` hook receives a single object containing:
 
-* `element` is a DOM element for that section. Nodes of that view for a atom
-  should be appended to the `element`.
-* `buffer` is an array passed to the `html` hook instead of a DOM element.
-  The content for the atom should be pushed on that array as a string.
+* `element` is a DOM element for the atom's content to be rendered.
 * `options` is the `cardOptions` argument passed to the editor or renderer.
 * `env` contains information about the running of this hook. It may contain
   the following properties:
   * `env.name` The name of this atom
-  * `env.remove()` remove this atom. This calls the current mode's `teardown()`
-    hook and removes the atom from DOM and from the post abstract.
-    the instance to edit mode.
 * `text` is the textual value used to display to the user.
 * `payload` is the payload for this atom instance. It was either loaded from
   a Mobiledoc or set when the atom was inserted.
-
-Additionally, *renderers may offer the ability to configure a non-standard
-hook name at runtime*. An example would be having the DOM renderer called with
-an option specifying the hook name `mobile-placeholder`. This allows for
-variants of an atom in different situations.