Add new attribute 'IMAGEKIT_CACHE_TIMEOUT'

[chrismaille]

In some projects the default timeout from caches are not very high - by choice or by Django default (5 minutes).

I believe this new attribute can give more control on default timeout behavior for imagekit.

Thank you for the time evaluating this PR.

[vstoykov]

Hello @chrismaille and thank you for your contribution.

In the past there was a discussion between me and the owner of the project about such settings. My opinion was that there should be such a settings and not relying on IMAGEKIT_CACHE_BACKEND, because you can use the same backend but just use different timeouts.

As I see now I'm not the only person who thinks that this something that developers wants.

My only concern about your PR is that you need to move the config in imgekit.conf (https://github.com/chrismaille/django-imagekit/blob/develop/imagekit/conf.py) with some default and use it from there.

[vstoykov]

Hi @chrismaille because you are not interested in your PR, I've started a new one which will also introduce 1 day cache by default. If you wan you can look at it in #403 and tell your opinion