Added a simple validation for new app name

Currently there are no validation for app name. For example when user entered like ```Vendor;Product``` or like this ```Vendor§§Produc``` name, Laravel gives syntax error.

I added a simple validaton for this issue. And PR contains a little improvement for Composer's namespace about this issue.
Should be a space on either side of the exclamation mark.

```php
if ( ! $this->isValidAppName())`
```