Expose MiddlewareStack#unshift to environment configuration

[benpickles]

Although #6372 added #unshift to the middleware stack this is not exposed in the environment configuration files.

I wanted to be able to add basic auth to my app and ensure that it was the first middleware to be hit, although I could use #insert_before my stack differs between environments. This change ensures that the following is possible:

config.middleware.unshift Rack::Auth::Basic, 'Restricted Area' do |user, pwd|
  [user, pwd] == ['admin', 'admin']
end

[pftg]

To complete this PR need to add Changelog entry.

[benpickles]

Thanks, I updated the changelog.

[senny]

@josevalim I've seen you merged #6372 can you take a look?

[josevalim]

Thanks @benpickles / @senny / @pftg !