conditionally load the Rails Engine Rakefile for increased cmd power

For example,

__if no Rakefile detected__,
```shell
rake build                           
rake common:test_app[user_class]  
rake extension:test_app[user_class]
rake install                     
rake release                  
rake spec                       
rake test_app                
```

__if Rakefile detected__,
```shell
rake app:about                                    
rake app:assets:clean                       
rake app:assets:precompile            
rake app:clean                                   
rake app:clobber                               
rake app:db:bootstrap                      
rake app:db:create                           
rake app:db:drop                              
rake app:db:fixtures:load                
rake app:db:load_dir[dir]                
rake app:db:load_file[file,dir]         
rake app:db:migrate                        
rake app:db:migrate:status            
rake app:db:remigrate                    
rake app:db:rollback                       
rake app:db:schema:dump           
rake app:db:schema:load             
rake app:db:seed                           
rake app:db:setup                          
rake app:db:structure:dump         
rake app:db:version                      
rake app:deface:clean                 
rake app:deface:get_result[virtual_path]
rake app:deface:precompile                     
rake app:deface:test_selector[virtual_path,selector] 
rake app:doc:app                                     
rake app:log:clear                                    
rake app:middleware                              
rake app:notes                                        
rake app:notes:custom                          
rake app:paperclip:clean                      
rake app:paperclip:refresh                   
rake app:paperclip:refresh:metadata  
rake app:paperclip:refresh:missing_styles
rake app:paperclip:refresh:thumbnails
rake app:rails:template                       
rake app:rails:update                                 
rake app:routes                                       
rake app:secret                                       
rake app:simplecov                               
rake app:spec                                        
rake app:spec:models                         
rake app:spree:install:migrations                     
rake app:spree:reset_taxon_permalinks            
rake app:spree_api:install:migrations                 
rake app:spree_auth:admin:create                      
rake app:spree_auth:install:migrations                
rake app:spree_extension_name:install:migrations
rake app:state_machine:draw    
rake app:stats                                 
rake app:test                                   
rake app:test:recent                       
rake app:test:single                        
rake app:test:uncommitted            
rake app:time:zones:all                  
rake app:tmp:clear                           
rake app:tmp:create                         
rake build                                           
rake common:test_app[user_class]
rake db:create                                   
rake db:drop                                      
rake db:fixtures:load                         
rake db:migrate                                  
rake db:migrate:status                       
rake db:rollback                                  
rake db:schema:dump                       
rake db:schema:load                          
rake db:seed                                         
rake db:setup                                        
rake db:structure:dump                       
rake db:version                                     
rake extension:test_app[user_class]
rake install                                          
rake release                                        
rake spec                                             
rake test_app                                         
```