Proposed new core library: alert.el

[John Wiegley <jwiegley@gmail.com>]

alert.el is a library I've been using for the last four years to provide a
richer notification interface than `message'. The current version is here:

    https://github.com/jwiegley/alert/blob/master/alert.el

It supports many alert backends:

;;   fringe        - Changes the current frame's fringe background color
;;   mode-line     - Changes the current frame's mode-line background color
;;   gntp          - Uses gntp, it requires gntp.el (see https://github.com/tekai/gntp.el)
;;   growl         - Uses Growl on OS X, if growlnotify is on the PATH
;;   ignore        - Ignores the alert entirely
;;   libnotify     - Uses libnotify if notify-send is on the PATH
;;   log           - Logs the alert text to *Alerts*, with a timestamp
;;   message       - Uses the Emacs `message' facility
;;   notifications - Uses notifications library via D-Bus
;;   notifier      - Uses terminal-notifier on OS X, if it is on the PATH
;;   toaster       - Use the toast notification system

alert.el also allows highly configurable user-styles, for deciding when and
how various alerts are presented.

I'm not proposing Emacs be changed to use alert, only that alert be made
available to all package authors as a richer alternative to `message' or
`notifications-notify'.

I suggest core rather than ELPA because I'd like to encourage wider use of
such a facility; but I could see an argument for moving it into ELPA too.
Being in core would give justification for adding a section in the Elisp
manual on using alerts for rich notification. Or, we could start applying it
in existing core modules where it makes sense to do so.

John