Add extensions for TagHelperOutput to copy from TagBuilder.

- Tested out Merge, MergeAttributes, PullPrefixedAttributes and RestoreBoundHtmlAttribute.

#1319
inconsistency between this method (where the input is the `context`) and `MergeAttributes()` (where the input is a `TagBuilder` instance) doesn't make sense to me. should also merge the `@class` attribute value here. that means we consistently honour what earlier tag helpers targeting an element have done and merge what the current one wants to add.