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 SCREAMING_SNAKE_CASE for environment variables