Enhancement: console should be extensible

[robbieaverill]

As a developer I should be able to add my own commands for the console.

They may live in a user directory, e.g. ~/silverstripe-console/src or in a SilverStripe application folder, e.g. /var/www/silverstripe-console/src

Any configuration YAML files should be merged into the main configuration file before commands are loaded.

We could also search through all vendor/ folders for a ssconsole.yml file to be able to pick up multiple modules installed with composer.

The user directory configuration (e.g. ~/ssconsole.yml) should take priority over custom modules, which should be loaded on top of core commands.

I don't imagine any way initially of being able to modify the core ssconsole behaviour, although we could probably use symfony console events to provide some level of abstraction if we need to.

[robbieaverill]

cc @silverleague/console for feedback

[robbieaverill]

Not required for initial release, so changing to 1.1.0 milestone