Create a more designed Menu Dropdown Button

[jguffey]

I had a similar use case on a project in the past, so I think it would be useful to have a dropdown that acts as a button.

Mostly it's just an example of what one can do with this library, but it might not be immediately apparent how to create one of these.

I also added yarn.lock to the .gitignore for those of us that want to use yarn.

[jguffey]

Yes! It's just an example outlining something I'm not is readily apparent - that you can use this plugin with buttons instead of fields. Feel free to reject this PR if it doesn't provide enough value, no hard feelings!