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 become a "shim" -- gracefully defer to Nokogiri by refusing to
load itself.

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 constants and singleton methods as `::Nokogumbo`:

  - `Nokogumbo.parse()` will be provided as `Nokogiri::Gumbo.parse()`
  - `Nokogumbo.fragment()` → `Nokogiri::Gumbo.fragment()`
  - `Nokogumbo::DEFAULT_MAX_ATTRIBUTES` → `Nokogiri::Gumbo::DEFAULT_MAX_ATTRIBUTES`
  - `Nokogumbo::DEFAULT_MAX_ERRORS` → `Nokogiri::Gumbo::DEFAULT_MAX_ERRORS`
  - `Nokogumbo::DEFAULT_MAX_TREE_DEPTH` → `Nokogiri::Gumbo::DEFAULT_MAX_TREE_DEPTH`

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 avoid
the "shim" behavior. This escape hatch might be unnecessary, but this
change is invasive enough to make me want to be cautious.

In "shim" mode, `Nokogumbo.parse()` and `.fragment()` will be
forwarded to the Nokogiri implementation. The `Nokogumbo::DEFAULT*`
constants will always be defined, but when in "shim" mode will be set
to the `Nokogiri`-provided values.

Nokogumbo will emit a single warning message at `require`-time when it
is in "shim" mode. 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).

I did not include deprecation warning messages in `Nokogumbo.parse`
and `.fragment`. If you feel strongly that we should, let me know.

lib/nokogumbo.rb

@@ -1,17 +1,38 @@
 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"))
 
-module Nokogumbo
-  # The default maximum number of attributes per element.
-  DEFAULT_MAX_ATTRIBUTES = 400
+  warn "NOTE: nokogumbo: Using Nokogiri::HTML5 provided by Nokogiri. See https://github.com/sparklemotion/nokogiri/issues/2205 for more information."
 
-  # The default maximum number of errors for parsing a document or a fragment.
-  DEFAULT_MAX_ERRORS = 0
+  module Nokogumbo
+    def self.parse(*args)
+      Nokogiri::Gumbo.parse(*args)
+    end
 
-  # The default maximum depth of the DOM tree produced by parsing a document
-  # or fragment.
-  DEFAULT_MAX_TREE_DEPTH = 400
+    def self.fragment(*args)
+      Nokogiri::Gumbo.fragment(*args)
+    end
+
+    DEFAULT_MAX_ATTRIBUTES = Nokogiri::Gumbo::DEFAULT_MAX_ATTRIBUTES
+    DEFAULT_MAX_ERRORS = Nokogiri::Gumbo::DEFAULT_MAX_ERRORS
+    DEFAULT_MAX_TREE_DEPTH = Nokogiri::Gumbo::DEFAULT_MAX_TREE_DEPTH
+  end
+else
+  require 'nokogumbo/html5'
+  require 'nokogumbo/nokogumbo'
+
+  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 depth of the DOM tree produced by parsing a document
+    # or fragment.
+    DEFAULT_MAX_TREE_DEPTH = 400
+  end
 end