We also make `--help` a non-early arg so it allows for the plugins to
register their options before printing the help message. The options
themselves are stored in a separate struct inbetween them being
registered and them being forwarded to the plugin. Currently only
supports string options.
Signed-off-by: Christian Decker <@cdecker>
Also includes some sanity checks for the results returned by the
plugin, such as ensuring that the ID is as expected and that we have
either an error or a real result.
