run gofmt on save

:warning: WIP :warning:

Still experimenting with the best way to run gofmt from within Atom. Since it's async, shelling out and using stdin/stdout were causing the buffer to need resaving again. Currently trying replacing the file underneath Atom to see how that goes.

- [x] Make gofmt on save optional
- [x] Add a command
- [ ] Make sure any errors running gofmt are handled in a sane way
- [ ] test it out a LOT