Allow to set the notification expiry

[huafu]

Allows to define the notification expiry on linux (when using notify-send).
By default it's set to 0 so the behavior isn't changed.
As added in readme, here is an example:

set -U __done_expire_time 3000 # 3 seconds

[bessey]

Would love this, really annoying to have to clear your notifications from all the accumulated long running commands this logs

[franciscolourenco]

@huafu thank you for the PR! Only now had time to have a look at it, sorry. 2 ideas:

1. Make the default 3000 (it sounds like a sane default behavior for this application)
2. Name option _done_notification_duration (a bit more generic) while still explaining in the README that it is currently only working in linux and is ignored by Ubuntu's Notify OSD and GNOME Shell.

What do you think?

[huafu]

@franciscolourenco done, except I did NOT list GNOME Shell since it DOES work in Gnome Shell (tested with GNOME Shell 42.9)

[franciscolourenco]

Thank you for the contribution ❤️

[franciscolourenco]

Available in https://github.com/franciscolourenco/done/releases/tag/1.18.0