Ability to customize method names without creating a custom router

Creating a custom router just to change the method name in the URI is too verbose, specially if you have many custom actions in a ViewSet and you only to customize single method.

This patch would allow developers to customize the default method name with something else by passing a custom_method_name as a part of list_route and detail_route decorators.
Let's go with... "If you do not want to use the default URL generated for your custom action, you can instead use the `url_path` parameter to customize it."