Add extensions for TagHelperOutput to copy from TagBuilder.

- Tested out Merge, MergeAttributes, PullPrefixedAttributes and RestoreBoundHtmlAttribute.

#1319
`whos <see cref="KeyValuePair{string, string}.Key"/> starts` -> `with <see cref="KeyValuePair{string, string}.Key"/> starting`
 - "whos" isn't a word
 - "whose" is also a poor idea because it personifies the key value pairs