[Proposal] Allow workbench stability to be set

As a Developer
When working on a dev version of Laravel and making a Laravel Package using workbench I would like to still use php artisan workbench without getting the error that matching stability level not found
So I can continue making Laravel packages for a project even when using a dev version of Laravel

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This way Workbench allows the stability option and more importantly continues to work when 
in transitions like L4 to L5.

this is related to this proposal https://github.com/laravel/framework/pull/6052


Thanks
Default MUST be dev at the moment otherwise laravel 5 is not installable.