Make it possible to initialize meta_request in an initializer

[abuisman]

Inspired by rack-mini-profiler I wanted to be able to conditionally enable and disable meta_request through environment variables.

We have a few tools for benchmarking pages and including them all each time we run our development server is not what we want. With my changes you can make a custom initializer and do this:

  if ENV['USE_RAILS_PANEL']
    require 'meta_request'

    # Initialize manually since we skipped it in the Gemfile
    MetaRequest::initialize!(Rails.application)
  end

The condition can be anything you want really.

I also added some documentation on the matter in de README.

I wasn't sure if you'd even be interested to merge this into the project, so I haven't written the tests yet. Also I think you know better what you want to do with those.

My take is that you can create some sort of group in the rails project under test/functional and then conditionally run using the 'normal' group and the one where meta_request has require: false and use an initializer in that case that does what my example above does.

Anyway. Let me know what you think.