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.
Yes most of the time, but ```Illuminate\Console\Command::execute``` returns mixed. So AppNameCommand can return a semantic boolean value. You suggestion is not sufficient to me.