Explore using `template` as the default context node

[flavorjones]

See https://github.com/flavorjones/nokogiri-html5-inference for the full context

We'd need to see if there is breakage downstream in the sanitizer libraries:

• https://github.com/rgrove/sanitize
• https://github.com/flavorjones/loofah and https://github/rails/rails-html-sanitizer

and if there is breakage, determine how to mitigate it.

See related:

• context node introduced in v1.17 by Fix fragment quirks mode, and make it easier to specify fragment context #3246
• discussion of default body behavior in:
  • [bug] DocumentFragment parse behavior wrt body tag #3023
  • HTML5 is unable to parse a fragment with table contents tags #3536

cc @stevecheckoway