fix(parent/child): Add host argument to parent and child factories.

[au-z]

Hi @smalluban!

Here's a totally unsolicited (but helpful) feature which you might find complementary to the existing parent/children factories...

For some complex components which reference parent and child elements based on element context, access to the host element is helpful. This can be done without a breaking change and creates some options for advanced component interactions.

For example, to get a parent which exposes a particular property name:

const MyChildComponent = define({
  tag: 'my-child-component',
  context: '__ctx__',
  contextProvider: parent((el, {context}) => el.hasOwnProperty(context)),
  // ...
})

Another use case:

const ChildElementCounter = define({
  tag: 'child-element-counter',
  childTag: '',
  children: children((el, {childTag}) => el.tag.toLowerCase() === childTag),
  // ...
})

Have a look and let me know what you think. Thanks! 👍

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[coveralls]

Coverage remained the same at 100.0% when pulling cd2259dd34db9eb14d7654ee47493a5d847be561 on auzmartist:parent-child-context into 92fc3660c1b0d18205e144bc067dc5d6f56091ac on hybridsjs:master.

[au-z]

Alright, I've added a couple more tests and added to the API docs. Ready for re-review @smalluban. 👍

[smalluban]

Nice work, thanks!