Use the Gumbo HTML5 parser instead of Nokogiri.

This change is EXPERIMENTAL and should not yet be used in production
environments or with untrusted input!

Note that quite a few tests currently fail due to parsing differences
between the Nokogiri parser and Gumbo.
Merge branch 'dev-3.0.0' into gumbo
Move REGEX_DATA_ATTR and REGEX_PROTOCOL into the CleanElement transformer.
Require Nokogumbo ~> 1.1.5.
Require Minitest ~> 5.3.4.
Add a history entry about Gumbo.
Mention Gumbo in the readme.
Remove :output and :output_encoding.
Use require_relative.
Move sanitize/config.rb to sanitize/config/default.rb.

For consistency.
Always traverse top-down; don't skip siblings added during traversal.

This removes the :transformers_breadth and :transformers_depth configs, which were poorly-named. All transformers now perform top-down
traversal.

This also fixes a bug where sibling nodes added by a transformer during
traversal were never traversed.

Fixes #90
Fixes #91
Bring the readme up to date.