Rework the prompt for the destination dir

Rework the prompt to improve the user experience.

Instead of asking for the target directory name, the generator asks:

    Would you like to create a new directory for your project? (y/N)

The default answer is
  - "no" for `yo loopback`
  - "yes" for `slc loopback`

The name of the directory created will be always the same as the project name.

See #56 for more details.

/to @raymondfeng please review