Allow running tasks without requiring all/any of rails (#199)

* Allow running tasks without requiring all/any of rails

- Remove initial action mailer require, per #60 (comment)
- Add an ENV var that allows you to not do any additional requires here, so that it's all handled by the Rails app

* Update README.md

README.md

@@ -595,12 +595,14 @@ Note: this plugs in the given webserver directly into rack, it doesn't use any `
 
 ### Excluding ActiveRecord
 
-By default, derailed will load ActiveRecord if the gem is included as a dependency.  It is included by default, if you just include the `rails` gem.  If you are using a different ORM, you will either need to only include the `railties` gem, or set the `DERAILED_SKIP_ACTIVE_RECORD` flag.
+By default, derailed will load ActiveRecord if the gem is included as a dependency.  It is included by default, if you just include the `rails` gem.  If you are using a different ORM, you will either need to only include the `railties` gem, or set the `DERAILED_SKIP_ACTIVE_RECORD` environment variable.
 
 ```
 $ DERAILED_SKIP_ACTIVE_RECORD=true
 ```
 
+Alternatively, use the `DERAILED_SKIP_RAILS_REQUIRES` environment variable to have derailed not require any Rails gems. Your app will then need to require them as part of its boot sequence.
+
 ### Running in a different environment
 
 Tests run against the production environment by default, but it's easy to

bin/derailed

@@ -80,9 +80,10 @@ class DerailedBenchmarkCLI < Thor
       require 'bundler/setup'
 
       begin
-        if ENV["DERAILED_SKIP_ACTIVE_RECORD"]
+        if ENV["DERAILED_SKIP_RAILS_REQUIRES"]
+          # do nothing. your app will handle requiring Rails for booting.
+        elsif ENV["DERAILED_SKIP_ACTIVE_RECORD"]
           require "action_controller/railtie"
-          require "action_mailer/railtie"
           require "sprockets/railtie"
           require "rails/test_unit/railtie"
         else