Add default blueprint

[BrianSipple]

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While doing some development on a local fork of ember-radio-button, I found the need to npm link it into a consuming application. The only way to have the addon's installation process triggered when doing this is for ember g <addon-name> to be run within the consuming project, so I generated a default blueprint for enabling that.

I figured I'd contribute it back while I was at it -- for anyone else with the same use case 😄 .