Add a new make:exception command

[jorgemudry]

What does this Pull Request Do?

It fixes the issue #76

How should this be manually tested?

1. Clone the repo from my fork:

git clone https://github.com/jorgemudry/lumen-generator.git;
git checkout feature_new_make_exception_command;

2. Change the name property in the composer.json file:

• Sorry, I don't know a better way to do this in order to use the package locally. If anyone knows a better way, please tell me :)

"name": "jorgemudry/lumen-generator",

3. In a new lumen project, add set up the local repository to the package in your composer.json:

    "repositories": {
        "dev-package": {
            "type": "path",
            "url": "../lumen-generator",
            "options": {
                "symlink": true
            }
        }
    }

4. Add the package to your project:

composer require jorgemudry/lumen-generator

5. Inside of your bootstrap/app.php add this:

$app->register(Flipbox\LumenGenerator\LumenGeneratorServiceProvider::class);

6. Execute the command to create a new exception:

php artisan make:exception MyPlainException

7. You should end up with a new file inside of the folder looking like this:

<?php

namespace App\Exceptions;

use Exception;

class MyPlainException extends Exception
{
    //
}

Any extra info?