Add extensions for TagHelperOutput to copy from TagBuilder.

- Tested out Merge, MergeAttributes, PullPrefixedAttributes and RestoreBoundHtmlAttribute.

#1319
nit: hard to parse. can this be wrapped to place the entire `.Where()` call on a single line? e.g.
``` c#
  var prefixedAttributes = htmlAttributes
    .Where(attribute => attribute.Key.StartsWith(prefix, StringComparison.OrdinalIgnoreCase))
    .ToArray();
```