Add gofmt + goimports Support

This PR picks up where https://github.com/atom/language-go/pull/3 left off by removing the snippets, and then ensuring [goimports](https://github.com/bradfitz/goimports) will work as a substitute for `gofmt`.

* :new: Support `goimports`
* :memo: Enable formatting on save by default
* :memo: Rename the menu item to "Format File"
* :lipstick: Remove tab options, which are no longer part of `gofmt` or `goimports`
* :new: Display `gofmt` errors, highlighting the line number(s) of the error(s)