Minor config prompt improvements

Fixing a few issues I notices running cookiecutter under Python 3.
- Now when the user issues "ctrl-C" the program will suppress the traceback from the ``KeyboardInterrupt``.

- ``prompt_for_config`` no longer encodes the string to bytes before passing it to ``input``. Before the prompt was printed to the console as ``b'full_name (default is "Your full name")? '``

- Suggests a cleaner look for the prompt á la [bower](http://bower.io/)/[npm](https://www.npmjs.org/).
    - Before: ``full_name (default is "Your full name")? ``
    - After: ``[?] full_name: (Your full name) ``

- Also adds a test case for when ``KeyboardInterrupt`` is raised and ``SystemExit`` is thrown in return. Note that I only got the *test* to work in Python 3, perhaps someone with more experience around mock can look into that.