This tells the plugin both the `lightning-dir` as well as the
`rpc-filename` to use to talk to `lightningd`. Prior to this they'd
had to guess.
Signed-off-by: Christian Decker <decker.christian@gmail.com>
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>