Add --source flag back to hugo new command

v0.15 had it, but I removed it inadvertently for 0.16-DEV
in commit 00d04774 in PR #1652.

Also add directory bash-completion for `--source`
for `hugo convert`, `hugo list` and `hugo new`.

See #1624, #1589

commands/convert.go

@@ -76,6 +76,7 @@ func init() {
 	convertCmd.PersistentFlags().StringVarP(&outputDir, "output", "o", "", "filesystem path to write files to")
 	convertCmd.PersistentFlags().StringVarP(&Source, "source", "s", "", "filesystem path to read files relative from")
 	convertCmd.PersistentFlags().BoolVar(&unsafe, "unsafe", false, "enable less safe operations, please backup first")
+	convertCmd.PersistentFlags().SetAnnotation("source", cobra.BashCompSubdirsInDir, []string{})
 }
 
 func convertContents(mark rune) (err error) {