Revise revision-data plugin approach & ecosystem #477

The team has concluded that the current approach of having a single revision-data plugin with data generators is inferior to having a family of revision-data plugins that you can choose from.

The reasons for this are 1) consistency (why have to learn a data generator API when you already know how to use/create plugins, 2) flexibility (data generators all have to run during the same hook in the current plugin, while dedicated plugins could execute the hook(s) that is most suitable).

Action items:

• Extract ember-cli-deploy-git-revision-data plugin (https://github.com/achambers/ember-cli-deploy-git-revision-data) (@achambers)
• Move ember-cli-deploy-git-revision-data plugin toember-cli-deploy org and grant npm privileges (@achambers)
• Extract ember-cli-deploy-md5-revision-data plugin to repo in ember-cli-deploy org
• Document expected deploy context keys/format for making your own revision-data plugin
• Update lightning-pack to use git plugin
• Deprecate the ember-cli-deploy-git-revision-data plugin