Integrate elixir 1.6 code formatter

[timmhirsens]

see:

http://devonestes.herokuapp.com/everything-you-need-to-know-about-elixirs-new-formatter

if editor.formatOnSave is set to true, we should run the formatter on save.

Maybe we can even provide a formatting extension point as described here:

https://code.visualstudio.com/blogs/2016/11/15/formatters-best-practices

[securingsincity]

I can take this one.

[dylan-chong]

whats going on with this? i notice that the current formatOnSave does not respect the rules from mix format