Profiles

Related doc: https://gist.github.com/tsloughter/1e16d70268182db1415a
Keeps working. This has worked for a long time just hasn't been the template. rebar3 checks if `relx` is defined in the config and passes that to `relx` if so. If it isn't then `relx` will look for `relx.config` because it doesn't get passed a config.