Added simple support for allow custom_filtering

[metazet]

In some cases we need to add custom filtering to our tastypie resources, but without adding to Resource.Meta.filtering. With this PR you can add all custom filtering to Resource.Meta class in format as the following:

custom_filtering = {
    'custom_field': {
        'dataType': 'string',
        'required': False,
        'description': 'Filtering custom field'
    },
}

[fradeve]

I've been searching for something similar for a while, hope this will get merged soon!
Personally tested on our Tastypie API with 50+ endpoints and 900+ tests, and everything works perfectly!

👍