client/as-builder: fix parser options

1. Use G_OPTION_ARG_FILENAME for all directoris params

2. Add additional argument description for bash-completion usage
   print to console --option-dir=DIR and something

Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>
bash-completion: add appstream-builder completion

Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>