utils/pb-plugin: Advertise pb-plugins to discover server

Update the pb-plugin utility to create a 'plugin' pb-event when it
successfully scans or installs a pb-plugin.
To aid invoking pb-plugin from pb-discover there following two commands
are updated:
 - `pb-plugin scan` can now be called for a single directory.
 - `pb-plugin install` now has an optional "auto" argument that will
 skip asking for confirmation before installation.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>