Add --extend, extend an existing project

This adds an `--extend` boolean flag that will
  cause to extend existing JSON files with the
  latest version from the template.

The `--extend` mode is not perfect, it will
  overwrite some changes you don't want to keep.

This means it's expected that the author uses git add -p
  to pick the changes they want and leave the rest.

This change also adds

 - better pretty printing
 - do not prompt for description on second scaffold
 - refactor .jshintrc to two spaces for JSON consistency