Allow to get a specified command from console Kernel

Summary:
Artisan::all() allows to get all the commands registered
in Nasqueron\Notifications\Console\Kernel, ie all custom
commands, in addition to the framework commands.

Currently, the logic to get one specified command lived
on our test suite. We move this to the Kernel class.

Test Plan: New unit tests for Kernel::get and Kernel::getByClass

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D795