Support dynamically changing goimports -local flag value from the current directory

[zchee]

What

Support dynamically changing goimports -local flag value from the current directory.

References

• https://github.com/zchee/nvim-go/blob/master/pkg/command/fmt.go#L76-L79
• https://github.com/golang/tools/blob/master/imports/fix.go#L31-L34
• https://github.com/golang/tools/blob/master/cmd/goimports/goimports.go#L50