[Command] Add option to configure project directory

[Command] Only set project directory if specified

[Specs] s/current working dir/project dir

[Command] Restore call to super in validate!

[Command] Make project directory parameter clearer

[Rubcocop] Fix argument alignmend in command.rb

[Command] Move project directory logic to its own module
Merge pull request #2184 from CocoaPods/config-project-directory

[Command] Add option to configure project directory