Support optional configuration target directory.

Begin to address making the configuration file generation a public API.
In other projects, it is desirable to leverage the existing RSpec
initialization generator. Allowing for the ability to target a different
location other than the current working directory.

Turn existing hard coded choices into configuration options:

- destination directory to copy files into

  Make default destination of the current working directory explicit.

- stream to report activities performed

  Allows the caller to decide if/where output should be displayed.

- location to read the initialization files from

  In order to achieve this, the existing template files needed to be moved
  into a structure which mirrored what was created.

  To keep the output consistent, remove the base directory from the
  display.

- Refactor initializer to be more configurable.

  Instead of special casing all of the files to create, generalize the
  method used to copy them, so that new files can quickly be added later.

  Move standard initialization files to constants.
Merge pull request #1563 from rspec/update-project-initializer

Update project initializer with greater control of how it works