--includeAngularDemo option does not work.

Environment

• CLI: 6.5.0
• Cross-platform modules: 6.2.3
• Android Runtime: 6.2.0
• iOS Runtime: 6.2.0
• Plugin(s): none

Describe the bug
When tns plugin create, --includeAngularDemo option does not work.

To Reproduce

$ tns plugin create --pluginName=foo --username=bar --includeTypeScriptDemo=y --includeAngularDemo=n ./nativescript-foo

Expected behavior

$ ls ./nativescript-foo/demo-angular
ls: cannot access './nativescript-foo/demo-angular': No such file or directory

But the directory exists.

Sample project
none

Additional context
#4616
https://github.com/NativeScript/nativescript-cli/blob/936cdff0813c4e6074ef1b2f3e5ec144c1070fcf/lib/commands/plugin/create-plugin.ts#L141
At this line, includeDemoOption is "y" or "n" then shouldIncludeDemo is always true.

從 lib/commands/plugin/create-plugin.ts 的第 141 行附近開始，使用 --includeAngularDemo=n 重現回報的 tns plugin create command。檢查 includeDemoOption 和 shouldIncludeDemo 的解讀方式。完成的定義是：當選項為 n 時，該命令不建立 demo-angular，同時保留對 y 的預期行為。