Creating a class to handle preparing ARGV.

Before the `AppGenerator` is started, `ARGV` needs to be modified to correctly account for some things (like showing the version). I'm extracting these out into their own class, and moving this class inside of the same file that defines `AppGenerator`. 

Right now, the preparation done by the `AppPreparer` class is done inside of the `commands/application.rb` file and it is thrown together in a hacky manner. I think instead of putting this logic in the `commands/application.rb` file (which in my opinion should only make a call the requisite classes), the `AppPreparer` should be extracted some place which is reasonably close to the `AppGenerator` class.