added support for range operator

[gartmeier]

I want to add support for the range operator to django-rql, but I'm a little lost. I need help understanding the terminology (comp, logical), and I may have put everything in the wrong places.

The goal is to allow the following query string generated by javascript-rql:

const filter = {
  age: {
    $range: {
      max: 5,
      min: 9,
    },
  },
};
 
rql(filter); //'range(age,9,5)'

Please have a look and let me know if I need to change something or add additional tests.

[CLAassistant]

All committers have signed the CLA.

[ffaraone]

Thank you @gartmeier for your contribution! Sorry if we are late in reviewing your contribution, we are very busy finishing the new release of our product. As soon as we can we will tell you something.