Add noContent property to Node

[alanpoulain]

It can be useful to specify that an extension has no content at all, for instance when parsing a HTML node containing multiple tags (I added an example for a <blockquote> containing a <cite> tag).

In JS, it can be done by not using the content key:

export const BlockquoteCite = Node.create({
  name: 'blockquoteCite',
  group: "block",

  parseHTML() {}

In this PR, I propose to add a $noContent static property to Node to specify this behavior.
When set to true, the content key is not added to the array.

[alanpoulain]

@timoisik Could you have a look at it? We need it for our project (we use a JSON version polluted by this content key).