Add exceptions collection

[kristoferjoseph]

When enhance renders it looks for the backing server template for every custom element on the page in the elements collection. If no element definition is found it throws an error.

We need to add an exceptions collection to support a case where there are custom elements on the page that you do not want enhance to try to expand.

Proposed enhancement:

const html = enhance({
  elements: {
    'enhance-header': EnhanceHeader
  },
  exceptions: [
    'random-element'
  ]
})

[kristoferjoseph]

OR we could console.warn instead of throwing, which might be the correct thing to do in order to cut down on the amount of work the user has to do.