Allow for conditional QuerySet.update_or_create()

[Joshua Cannon]

QuerySet.update_or_create() is great since it lifts the burden of thread-safety and repeated code from the client. However there exists some scenarios where the "update" logic should be hidden behind a condition. (E.g. only update if the value of a DateTimeField is less than somevalue). There isn't much help for clients in these scenarios, with the best solution being to copy+paste QuerySet.update_or_create() and add a condition to the if created: expression.

The condition would most likely be a callable that accepts one argument, the gotten instance.

The solution should be trivial, either:

1. Add a "condition" parameter to update_or_create() (along with logic to make the feature backwards compatible, likely checking if the value is callable with one argument or not, since "condition" could be a field)
2. Add another method, and re-write update_or_create() in terms of the new method with a condition that is always true.