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

parent #543 
Also implement part of #541 , that is, if `--wwwroot-out` doesn't have a value, we don't create public folder in packed image.
`can only be used when` --> `can be used only when`