Refactor some method into public postEditor methods

[mixonic]

This begins the exposure of a public API programmatically interacting with the editor. For example:

let editor = new ContentKit.Editor(someElement);

$('button').append(document.body).on('click', () => {

  let markerRange = editor.cursor.offsets;
  let markup = editor.builder.createMarkup('strong');
  editor.run((postEditor) => {
    postEditor.removeMarkupFromMarkers(markerRange, markup);
  });

});

While the editor and the cursor may create post abstract objects or read data about the current state of the editor, any modifications must be make through the postEditor interface. The runloop of the postEditor ensures that no matter what operations are performed only a single render and update are performed.

TODO:

• On the postEditor, this.rerender() should be this.scheduleRerender(). Likewise for didUpdate
• Docs
• The internals of handleNewline must be migrated to postEditor
• Operations during reparse must move to the postEditor actually, reparse should be going away now that we control delete/enter. Should discuss with @bantic.
• loadModel should become part of the editor dropped loadModel, we don't really have an immediate need.
• rerender itself should be moved to the postEditor, and the renderer should be passed to the postEditor instance. seems coupled to loadModel concerns. Will revisit.

[mixonic]

Rebase and merge with @bantic's latest, also handling of newlines is ported. Moving through the other items.

[bantic]

Elsewhere in the tests we use # for instance method abbreviation and . (dot) for class method abbreviations. To stay in sync with those this should say "#isBlank returns ..."

[bantic]

This feels like inside baseball. The italic command should never be exec-able if there isn't a selection — if the right/left render nodes are not present then something else is wrong, or the command is being exec'd when there's no selection to apply markup to.

[mixonic]

Ah. I was able to create a failure here in chrome with some wacky selection drags. We should look at it together, I'll make a ticket.

[bantic]

👍 this is great