Add extensions for TagHelperOutput to copy from TagBuilder.

- Tested out Merge, MergeAttributes, PullPrefixedAttributes and RestoreBoundHtmlAttribute.

#1319
- why does this name remain unchanged?
- FYI I kind-of liked @yishaigalatzer's suggestion to split this in two -- say `FindPrefixedAttributes()` and `RemovePrefixedAttributes()` (or perhaps `RemoveRange()` if Linq doesn't provide that as an `IEnumerable` extension)