v13. Persist local modules in target buffers. Make `erc-modules' custom
`:set' function local-module aware. Add helper for defining toggles in
`define-erc-module'. Add more tests.

"J.P." <jp@neverwas.me> writes:

> Another possible UX annoyance is that local toggles, like
> `erc-sasl-mode', don't currently work in target buffers at all. We
> should maybe wrap them in an `erc-with-server-buffer'.

Actually, nah. Some local modules may operate in target buffers.