Made theme description user changeable for underscores scaffold

[Vincent-Noben]

Hi,

The underscores.me website allows a Theme description. In the scaffold function it was hardcoded and not thus allowed as a argument.

I just added the hardcoded value as the default value, and thus, allow for it to be set as a cli parameter.

The reason that it was hardcoded in the first is that it implemented the theme_name and author in the description. And those variables can obviously not be used in the $defaults before wp_parse_args is run. As a solution I added those back in with sprintf. Also updated the help documentation above the function

Note: Untested as I don't know howto, nor have the time to compile a new wp ... phar ... thing ... whatever-it's-called 😅 But given the fact that is a very small change I assume the chanse of it breaking stuff rather small. 😉