Add '--wwwroot' and '--wwwroot-out' option to "kpm pack"

- '--wwwroot' specifies the name of public folder in project dir
- '--wwwroot-out' specifies the name of public folder in packed image
- '--appfolder' is replaced by '--wwwroot-out'
- If '--wwwroot-out' is not specified, the packed image doesn't contain a public
  folder
- If 'webroot' property is specified, use it as default value of
  '--wwwroot'
- "kpm pack" updates 'webroot' of project.json in the packed image