Simplify parameters for `scaffold post-type` and `scaffold taxonomy`

The synopses for these subcommands are massive:

```
$ wp scaffold
usage: wp scaffold _s <slug> [--theme_name=<title>] [--author=<full-name>] [--author_uri=<http-url>] [--activate]
   or: wp scaffold plugin <slug> [--plugin_name=<title>] [--activate]
   or: wp scaffold post-type <slug> [--singular=<label>] [--description=<description>] [--public=<public>] [--exclude_from_search=<exclude_from_search>] [--show_ui=<show_ui>] [--show_in_nav_menus=<show_in_nav_menus>] [--show_in_menu=<show_in_menu>] [--show_in_admin_bar=<show_in_admin_bar>] [--menu_position=<menu_position>] [--menu_icon=<menu_icon>] [--capability_type=<capability_type>] [--hierarchical=<hierarchical>] [--supports=<supports>] [--has_archive=<has_archive>] [--query_var=<query_var>] [--can_export=<can_export>] [--textdomain=<textdomain>] [--theme] [--plugin=<plugin>] [--raw]
   or: wp scaffold taxonomy <slug> [--singular=<label>] [--public=<public>] [--show_in_nav_menus=<show_in_nav_menus>] [--show_ui=<show_ui>] [--show_tagcloud=<show_tagcloud>] [--hierarchical=<hierarchical>]  [--rewrite=<rewrite>] [--query_var=<query_var>] [--textdomain=<textdomain>] [--post_types=<post_types>] [--theme] [--plugin=<plugin>] [--raw]

See 'wp help scaffold <subcommand>' for more information on a specific subcommand.
```

They're so long, they stop being useful for describing what parameters are accepted.

But more than that, I think most of the parameters aren't even very useful. For example:

 `--textdomain` is useful because it's used for all the label strings.

`--show_in_admin_bar` is less useful because it's only used once, so you might as well edit it in your code editor.