Add support for exfmt

[Nitin Gupta]

Please consider adding support for elixir code formatter: exfmt (https://github.com/lpil/exfmt). Recently exfmt seems to have implemented all features required for integration with emacs: lpil/exfmt#27

[Sebastian Kosch]

For anyone looking for this, M-x elixir-format does the job just fine, using the new formatter built into Elixir 1.6. I think this issue can be closed.