helpers: Fix golint issues

helpers: Fix golint issues

helpers/general.go:448:1: comment on exported function DiffStrings should be of the form "DiffStrings ..."
helpers/hugo.go:42:6: exported type HugoVersionString should have comment or be unexported
helpers/hugo.go:48:1: exported method HugoVersion.Version should have comment or be unexported
helpers/hugo.go:56:1: comment on exported method HugoVersionString.Compare should be of the form "Compare ..."
helpers/hugo.go:62:1: comment on exported method HugoVersionString.Eq should be of the form "Eq ..."
helpers/path.go:548:1: comment on exported function OpenFilesForWriting should be of the form "OpenFilesForWriting ..."
helpers/processing_stats.go:24:6: exported type ProcessingStats should have comment or be unexported
helpers/processing_stats.go:55:1: exported function NewProcessingStats should have comment or be unexported
helpers/processing_stats.go:59:1: exported method ProcessingStats.Incr should have comment or be unexported
helpers/processing_stats.go:63:1: exported method ProcessingStats.Add should have comment or be unexported
helpers/processing_stats.go:67:1: exported method ProcessingStats.Table should have comment or be unexported
helpers/processing_stats.go:83:1: exported function ProcessingStatsTable should have comment or be unexported