Add extensions for TagHelperOutput to copy from TagBuilder.

- Tested out Merge, MergeAttributes, PullPrefixedAttributes and RestoreBoundHtmlAttribute.

#1319
- should not use `Add` since it will throw if `Attributes` already includes an entry w/ this name (perhaps an earlier tag helper in line already passed a value through)
 - instead must make sure existing entry in `tagHelperOutput` is not overwritten:
``` c#
  if (!tagHelperOutput.Attributes.ContainsKey(attributeName))
  {
    var entry = ...;
    var originalAttribute = ...;
    tagHelperOutput.Attributes[originalAttribute.Key] = originalAttribute.Value;
  }
```
 - not sure whether this should also handle merging "class" attribute values. could for example use a refactored part of `MergeAttributes()`