* Markup API doc in Markdown style
- Specify `--markup markdown` option to YARD.
- Add and publish `.yardopts` file, instead of giving the option in `Rakefile`.
This `.yardopts` file allows to use the same options on <https://www.rubydoc.info/>.
- Rewrite the `Capybara.configure` method doc because it's broken in Markdown style.
- Fix `Capybara::Selector` doc in markdown style