Hi people,

I'd like to take this feature over, in case anyone cares. To summarize,
it initially stalled out because an underlying facility to support the
dynamic updating of rich UI elements wasn't available at the time. Most
of it has since been added, and the attached changes (once complete)
should fill in any remaining gaps.

Thus, I've gone ahead and integrated everyone's suggestions, for the
most part, with the only caveat being the feature won't be enabled by
default. Rather, there's an added step involved where a user must first

  (setopt erc-prompt #erc-prompt-format)

before ERC will consider the companion option that contains the actual
template (also called `erc-prompt-format'). Such indirection may be
regrettable from a UX standpoint, but I'd rather hold off on improving
things until we've brought batch processing fully into the fold and have
tuned it to perform respectably with ERC's default configuration.

For anyone unfamiliar, ERC will soon be needing to process incoming
messages in rapid succession all the way to insertion as fast as it can
manage. Like normal messages, these will also influence the state of UI
elements, like the prompt, the mode line, etc. Because such processing
will be foundational to ERC's basic operations going forward, it's
important to prioritize über alles. To that end, I'm hoping we can
revisit this feature again at some later date if folks end up wanting to
expand `erc-prompt' to accommodate format specifiers directly, as
originally envisioned.

Thanks.