Add config option to disable clear command

[binaryfire]

This allows the purify:clear command to be disabled if we're not using a unique cache store. It prevents devs from trying to manually clear old definitions and unknowingly nuking the entire app's cache. In-memory cache stores always get cleared eventually (eg. when upgrading to a newer version), so letting old definitions persist until then isn't really a problem.

It defaults to false if the config key isn't present, so it's backwards compatible.

[stevebauman]

Hmm I'm not sure I like a boolean config value to disable the command.

Maybe we should just add a warning (similar to the the default php artisan migrate command in production mode), that displays to the executor that it will wipe the entire cache if that driver is configured?

[binaryfire]

Ok no worries. Yeah that would definitely be better than nothing.

I'll probably still disable the command with some CommandStarting event hackery on my end.