CLI: Create Artisan Command for Listing Feature Usage

[dshafik]

Create an artisan command pennant:list to show all feature flags used in a project.

Acceptance Criteria

1. Should display a nice list/table of every feature flag definition (if it exists) with all of it's usages within the code
2. Should work for any Laravel project
3. Differentiate between flag names and class-based flags (color? glyph?)

Example output:

beacon-enabled . . . . . . . . . . .  app/Providers/AppServiceProvider.php
                app/Http/Controllers/IndexController.php
                app/Http/Controllers/Middleware/EnsureBeaconEnabled.php

BillingEnabled . . . . . . . . . . .  
                app/Http/Controllers/IndexController.php

                                                             Showing 2 flags