Add documented but not implemented feature so a user can insert a middleware at a specific point in the stack

[michaellennox]

What does this PR do?

Adds the ability for a user to insert a middleware at a specific point in the middleware stack using the insert method.

Where should the reviewer start?

First commit demonstrates failing integration test case. Second fixes it. Recommended to step through.

Any background context to be provided?

In the README of this project it is documented that a user can insert a middleware at a specific point in the stack using the .insert method.

Notable passage:

You can add your custom middleware with use, that push the middleware onto the stack, and you can also control where the middleware is inserted using insert, insert_before and insert_after.

Notable example:

class CustomOverwriter < Grape::Middleware::Base
  def after
    [200, { 'Content-Type' => 'text/plain' }, [@options[:message]]]
  end
end

class API < Grape::API
  use Overwriter
  insert_before Overwriter, CustomOverwriter, message: 'Overwritten again.'
  insert 0, CustomOverwriter, message: 'Overwrites all other middleware.'

  get '/' do
  end
end

This was added in c2e41f7 but it appears that the insert method was missed from the public API and only exposed on the internal middleware stack class.

Any specific implementation details?

I added the CHANGELOG entry under Fixes as it is 'fixing' a discrepancy between the documentation and the code but unsure whether it belongs best under this or under Features.

[dblock]

Perfect, merged.