Experimental: C-u M-x elpher-menu with history

When called with a prefix argument, the elpher-menu now displays
history items as well. That is, instead of showing each buffer in
Elpher or Eww mode, it shows those buffers, and their history (based
on elpher-history and eww-history).

Sadly, there are no buffers for these, therefore operating on them
does nothing meaningful: visiting them gets you the current
buffer (the last one of every history branch).

Ideally, we'd rework all the buffer commands so that the know what to
do with history entries: visiting the current buffer and hitting
"back" a few times, for example. Or cloning the current buffer and
doing that.

Then again, perhaps we should limit ourself to a much smaller set of
commands for elpher-menu-mode-map (and the doc string of
elpher-menu-mode).