Add: Custom message format spec for notification buffers

[phil-s]

• ement-notifications-message-format-spec: New user option.
• ement-notifications-room-button-separator: New user option.
• ement-notifications-log-to-buffer: Use them.

[phil-s]

There were a couple of different message specs in the code:

- (ement-room-message-format-spec "%o%O »%W %S> %B%R%t")
- (setq-local ement-room-message-format-spec "[%o%O] %S> %B%R%t"

I only observe the first of those formats in the rendered notifications, so maybe the code setting the other can actually be removed?

I've also added a couple of helper code comments like this for locating the event formatters:

;; Formatter definitions: (occur "(\\ement-room-define-event-formatter")

I found that a convenience to be able to call with C-x C-e, as it's not the easiest thing to locate the code rendering each of those format spec items, and I think newcomers would find it especially useful. The 'unnecessary' escaping \\e is there to prevent those comments themselves being displayed in the results.