Rewrite `printf` vs `echo` guideline

If not passing options, including escapes, or including variables (which
may contain escapes), `echo` will work consistently across systems. It
should not be broadly avoided as this simple and safe use-case is
common.

http://unix.stackexchange.com/questions/65803/why-is-printf-better-than-echo
Add first guidelines for Ember
Remove the partial guideline

* https://github.com/thoughtbot/guides/commit/a2428be0f85fa30c0c30dba8d5843e86ff1163f8#commitcomment-8468894
Use `ENV.fetch` instead of `ENV[]`

It's easy to forget to set environment variables (e.g. `AWS_SECRET_KEY`) on
staging or production. Using `ENV.fetch("AWS_SECRET_KEY")` ensures that missing
environment variables raise errors instead of just being `nil`.

Heroku boots the app on deploy, which runs the app's initializers. Often,
environment variables are set in an initializer. This combination means that if
an environment variable isn't set, an error will be raised and Heroku will abort
the deploy. This means users can never see a version of the app without the environment
variables set.