Relationships rule

[javierbrea]

It is desirable to add a new rule allowing to define allowed relationships for different elements.

The configuration for the rule could be like:

{
  "rules": {
    "boundaries/relationships": [2, {
      "default": "allow",
      "rules": [
        {
          "from": ["helpers"],
          "disallow": ["child", "parent"]
        },
        {
          "from": ["components"],
          "disallow": ["uncle"]
        }
      ]
    }]
  }
}

[javierbrea]

This proposed rule may have conflicts with the no-private one, because child elements, for example, would be considered private elements. See #198 for further info.

Maybe it could be implemented as advanced configuration of the no-private rule.