Consider providing default from_number

[ozandogrultan]

https://github.com/Hipo/django-sms-toolkit/blob/master/django_sms_toolkit/tasks.py#L11

We should consider getting from_number directly from DJANGO_SMS_TOOLKIT_SETTINGS which will let the user not being obligated to pass from_number=settings.DJANGO_SMS_TOOLKIT_SETTINGS["DEFAULT_FROM_NUMBER"] every time task is called.

Pointed by: @fergalwalsh

[yigitguler]

I think from_number can be customizable. We can make it optional.

[alicertel]

Providing from_number is optional if you are using TwilioMessageMixin. Don't u use the mixin?

[alicertel]

It can also be optional in send_sms task similar to the method in mixin.