Allow functional constructs to be chained with the QuerySet

[onyb]

The idea is to do something like:

SomeClass.objects.filter(val__lte=<foo>).value_list('val', flat=True).map(callable)

This needs to be spec'd properly, but some inspiration can be taken from:

• http://jrsinclair.com/articles/2017/javascript-without-loops/
• http://jrsinclair.com/articles/2017/indentation-is-the-enemy-less-complex-javascript/