Add custom display delay support

[pix106]

I added custom delay support, do you want to merge it ?

[demestav]

Thank you for taking the time to open this pull request. One thing I would do differently, is to put the delay logic in the template tag and then pass the value with json_script.

[pix106]

Hi,
I just did the same as the other global setting DJANGO_MESSAGES_DISPLAY_CLOSE_BUTTON.
Do you mean you would have added per-message delay, instead of a global settings ?
Global setting is what i wanted to add, as i'd like to adjust delay for different apps.
Both could be useful anyway IMO.

Locally i did move the default delay outside of template, which is a bit cleaner.
I'm not used to PR, idk what would happen to the PR if i rebased and force pushed to my fork :)