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 adding [goimports](https://github.com/bradfitz/goimports) support.

* :new: Support `goimports`
* :memo: Enable formatting on save by default
* :memo: Disable use of `goimports` by default (as it requires a go get code.google.com/p/go.tools/cmd/goimports)
* :memo: Rename the menu item to "Format File"