Rework the prompt for the destination dir

Rework the prompt to improve the user experience.

 1. Ask for the application name, propose the 1st argument or cwd as the
    default.

 2. If the current dir is the same as the app name, then assume the
    destination dir to be '.' and run the generator.

 3. Otherwise ask the user for the target directory, using the project
    name as the default value.

See #56 for more details.

/to @raymondfeng please review