[c-lightning] Plugins/passthrough for next version?

[Rusty Russell]

Hi all,

        With 0.6.1 rc1 tagged, it's time to look forward.  One area
which needs work is making payments and automation: we do pretty well as
a server, but we're lagging as an actual wallet.

To that end, I want to add "plugins", which the lightning-cli or
lightningd itself would pass through.  These could do their own
communication with c-lightning in the background).

The first would be "pay" which really doesn't need to be part of
lightningd (and is quite complex and growing more so); the lower-level
commands should be (made?) sufficient.  Another could be an autopilot,
which would connect and manage connections.  More custom ones should be
encouraged, too.

There's no reason these need be in C, either.  They'll need some support
to access commandline/config file options and other details, but that
can be easily implemented.

Feedback welcome!
Rusty.