Rename plugin to app

[bkeepers]

• Release create-probot-app
• Test that create-probot-app works
• Update https://github.com/probot/plugin-template (Rename plugin to app template#22)
• Merge Rename plugin => app probot#210
• Rename this repo to create-probot-app
• Deprecate create-probot-plugin

[bkeepers]

create-probot-app is released and I tested that it works.