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/list.go

@@ -26,6 +26,7 @@ func init() {
 	listCmd.AddCommand(listDraftsCmd)
 	listCmd.AddCommand(listFutureCmd)
 	listCmd.PersistentFlags().StringVarP(&Source, "source", "s", "", "filesystem path to read files relative from")
+	listCmd.PersistentFlags().SetAnnotation("source", cobra.BashCompSubdirsInDir, []string{})
 }
 
 var listCmd = &cobra.Command{