Issue deprecation warning for initializing the driver with a hash

[jferris]

Several users are registering custom webkit drivers:

Capybara.register_driver :quiet_webkit do |app|
  Capybara::Webkit::Driver.new(app, stderr: QuietWebkit.new)
end

When passing a hash, the globally configured options are not applied, which is confusing. Let's issue a deprecation warning and provide documentation for customizing the driver.

[twalpole]

Is this still valid?