Add blueprint listing in ember help generate

Refactor help API around `printBasicHelp` and `printDetailedHelp`.

`ember help` uses `printBasicHelp`

`ember help <command-name>` prints basic and detailed help.

Implement `printDetailedHelp` in the generate command to list out
available blueprints, grouped by package.
Add changelog entry
Merge pull request #952 from jgwhite/generate-list

ember help generate