Support encrypting content

[tonysm]

Added

• Introduces a new options API to rich text attributes, which allows us to set some of them as encrypted

---

closes #34

---

This PR allows passing the encrypted options to any rich text fields to ensure they are encrypted at rest. It should allow encryption on a per-field basis. Since we may have multiple fields on the same model (record), this will allow us to encrypt only the ones we want to.

Example usage:

use Illuminate\Database\Eloquent\Model;
use Tonysm\RichTextLaravel\Models\Traits\HasRichText;

class Message extends Model
{
    use HasRichText;

    protected $richTextAttributes = [
        'content' => ['encrypted' => true],
        'notes', // Not encrypted...
    ];
}

[tonysm]

Merged this. But if anyone has anything against it or any other API suggestion, feel free to comment.