Add configurable basic authentication

[shinofumijp]

I want to use basic authentication on letter_opener page. To write an initializer like below, users can set basic authentication on letter_opener page.

config/initializers/letter_opener_web.rb

LetterOpenerWeb.config do |config|
  config.authentication_enabled = true
  config.authentication_options = { user: 'authuser', pass: 'authpass' }
end

If documentations are needed, I'll write it on README or wiki.

[pseudomuto]

This is very old at this point and has no recent activity. I'm going to close it for now. Feel free to open a new issue/PR if this is still a concern.