Adding ability to specify default_order_field on ModelAdmin

[theunraveler]

Hello. This PR adds the ability to manually specify an ordering field, in case you'd like to use a field other than the first on in Model._meta.ordering. Instead, you can just do this:

class MyModelAdmin(SortableAdmin):
    default_order_field = 'created_at'

[jrief]

I am missing a reference in docs/source/usage.rstabout this new feature.

Could you please explain the use-case for this feature.

Thanks

[theunraveler]

Ah, upon closer review, you can override the default ordering with ModelAdmin.ordering, making this PR redundant. Thanks, and sorry for the noise.