Close quickfix window on successful fmt

Prior to 674bdbf40bc3a4ea6beb1ac44feff02d4d05695b, fixing `go fmt`
errors would cause the quickfix window to be closed. This commit
restores that functionality for cases in which we think we control the
contents of the quickfix window.
Merge pull request #64 from zenazn/close-quickfix

Close quickfix window on successful fmt