v3.1.0

• Fix a few minor issues after v3 release
• Move docgen sub-pkg to https://github.com/go-chi/docgen
• Move render sub-pkg to https://github.com/go-chi/render
• Add new URLFormat handler to chi/middleware sub-pkg to make working with url mime
  suffixes easier, ie. parsing /articles/1.json and /articles/1.xml. See comments in
  https://github.com/go-chi/chi/blob/master/middleware/url_format.go for example usage.