Add --markdown option to make:mail generator

[simonhamp]

Adds the necessary stubs and methods to support Markdown mailables in Lumen. Don't forget to composer require illuminate/mail tho!

Fixes #26

[joelhy]

Please add "use Symfony\Component\Console\Input\InputOption;" to fix "Class 'Flipbox\LumenGenerator\Console\InputOption' not found" error, and squash your commits.

And I will merge your pr.

[simonhamp]

@joelhy Fixed. Can you not squash commits through GitHub?

[joelhy]

Squash option found.