Add support for configurable timestamp format

Also changes default timestamp format from ISO8601 to classic 24-hour
format as this is assumed to be the format that most users would prefer.

And reintroduces strict but optional ISO8601 format.

This feature allows to easily add more timestamp formats in the future.

src/bash-completion/tio.in

@@ -69,7 +69,7 @@ _tio()
             return 0
             ;;
         -t | --timestamp)
-            COMPREPLY=( $(compgen -W "${opts}" -- ${cur}) )
+            COMPREPLY=( $(compgen -W "24hour 24hour-start iso8601" -- ${cur}) )
             return 0
             ;;
         -L | --list-devices)