Add extensions for TagHelperOutput to copy from TagBuilder.

- Tested out Merge, MergeAttributes, PullPrefixedAttributes and RestoreBoundHtmlAttribute.

#1319
- suggest adding a `<remarks/>` section to the doc comments that covers appending to existing `Content`.
- may also make sense for that comment to mention multiple tag helper scenarios
- `<remarks/>` should definitely make it clear this method may not be appropriate for all tag helpers