Add znotify for slow commands with support for auto-remove

[prashantv]

znotify: https://github.com/marzocchi/zsh-notify

example config:

source ~/.zsh_plugins/zsh-notify/notify.plugin.zsh
zstyle ':notify:*' command-complete-timeout 5
zstyle ':notify:*' error-title "Command failed (#{time_elapsed}s)"
zstyle ':notify:*' success-title "Command succeeded (#{time_elapsed}s)"
zstyle ':notify:*' error-icon "/Users/prashant/Pictures/warn.png"
zstyle ':notify:*' success-icon "/Users/prashant/Pictures/info.png"

this works well, but the notifications have to be cleared manually. both terminal-notifier and alerter support removing notifications when created with a group id. Ideally, once the terminal has focus, or another command is run, the old notification is removed automatically.