Adding ability to override defaults in config

[mansona]

This is only a proof of concept to see if you are happy with the idea, and if so we can extend it to all/any of the parameters you want to be able to set the defaults for in config.

The basic idea here is: you have a sensible default set for matchTriggerWidth on ember-power-select but I need this default to be different for all instances of ember-power-select across my codebase.

Using a trick similar to we use in ember-cli-notifications I have been able to test this locally with a linked ember-power-select. If you like the idea I can (try) implement some tests using https://github.com/tomdale/ember-cli-addon-tests.