feat: Nokogumbo detects Nokogiri's HTML5 API

Closes #170

A future version of Nokogiri will provide Nokogumbo's API (see
sparklemotion/nokogiri#2204). This change
will allow Nokogumbo to detect whether Nokogiri provides the HTML5 API
and whether to use Nokogiri's implementation or Nokogumbo's
implementation.

Some contractual assumptions I'm making about Nokogiri:

- Nokogiri will faithfully reproduce the `::Nokogiri::HTML5` singleton
  method, module, and namespace (including classes
  `Nokogiri::HTML5::Node`, `Nokogiri::HTML5::Document`, and
  `Nokogiri::HTML5::DocumentFragment`)

- Nokogiri will not provide a `::Nokogumbo` module/namespace, but will
  provide a similar `::Nokogiri::Gumbo` module which will provide the
  same public API as `::Nokogumbo`.

This change checks for the existence of `Nokogiri::HTML5`,
`Nokogiri::Gumbo`, and an expected singleton method on each. We could
do a more- or less-thorough check here.

This change also provides an "escape hatch" using an environment
variable `NOKOGUMBO_IGNORE_NOKOGIRI_HTML5` which can be set to force
Nokogumbo to use its own implementation. This escape hatch might be
unnecessary, but this change is invasive enough to make me want to be
cautious.

Nokogumbo will emit a single warning message at `require`-time when it
is uses Nokogiri's implementation. This message points users to
sparklemotion/nokogiri#2205 which will
explain what's going on and help people migrate their
applications (but is an empty placeholder right now).

lib/nokogumbo.rb

@@ -1,17 +1,27 @@
 require 'nokogiri'
-require 'nokogumbo/version'
-require 'nokogumbo/html5'
 
-require 'nokogumbo/nokogumbo'
+if ((defined?(Nokogiri::HTML5) && Nokogiri::HTML5.respond_to?(:parse)) &&
+    (defined?(Nokogiri::Gumbo) && Nokogiri::Gumbo.respond_to?(:parse)) &&
+    !(ENV.key?("NOKOGUMBO_IGNORE_NOKOGIRI_HTML5") && ENV["NOKOGUMBO_IGNORE_NOKOGIRI_HTML5"] != "false"))
+
+  warn "NOTE: nokogumbo: Using Nokogiri::HTML5 provided by Nokogiri. See https://github.com/sparklemotion/nokogiri/issues/2205 for more information."
+
+  ::Nokogumbo = ::Nokogiri::Gumbo
+else
+  require 'nokogumbo/html5'
+  require 'nokogumbo/nokogumbo'
 
-module Nokogumbo
-  # The default maximum number of attributes per element.
-  DEFAULT_MAX_ATTRIBUTES = 400
+  module Nokogumbo
+    # The default maximum number of attributes per element.
+    DEFAULT_MAX_ATTRIBUTES = 400
 
-  # The default maximum number of errors for parsing a document or a fragment.
-  DEFAULT_MAX_ERRORS = 0
+    # The default maximum number of errors for parsing a document or a fragment.
+    DEFAULT_MAX_ERRORS = 0
 
-  # The default maximum depth of the DOM tree produced by parsing a document
-  # or fragment.
-  DEFAULT_MAX_TREE_DEPTH = 400
+    # The default maximum depth of the DOM tree produced by parsing a document
+    # or fragment.
+    DEFAULT_MAX_TREE_DEPTH = 400
+  end
 end
+
+require 'nokogumbo/version'