This feature initially included a small omission in its lack of support
for left-sided time stamps. Apparently, they're popular enough to
warrant the additional complexity. The attached patch attempts to add
that support as well as fix a few related bugs.

It currently introduces two options:

  `erc-fill-wrap-margin-width'
  `erc-fill-wrap-margin-side'

Both are nil by default, but the second must be customized for users who
define their own `erc-insert-timestamp-function'.

Note that this variant behaves a little differently with regard to the
prompt, which appears in the left margin via `display' properties. The
option `erc-fill-wrap-width' controls the margin's starting width, which
defaults to either stamp width or prompt width: whichever's wider on
MOTD. The prompt is padded on the left and truncated on the right if
need be to conform to the margin. This look may take some getting used
to, but I think most will agree that it's preferable to the alternative,
which would see the prompt floating in no man's land, between the margin
and the "static center," where speaker labels are right-aligned.

As with the right margin, the left can also be adjusted in-session with
the command `erc-fill-wrap-nudge' and the keys `)', `_', and `+'.

On a related note, I'm also proposing we remove the `margin' Custom
:type choice for the option `erc-timestamp-align-to' (new in 5.6). It
was only ever tangentially related and doesn't really do much, and it
only really existed to service the needs of the internal minor mode
`erc-stamp--display-margin-mode'.

Thanks.