add short options, output filename

I think with short option names would be easier to use this program, but I think renaming `string` to `text` for `-t` might be a good idea, so it doesn't have to have `-S` for shape.

I removed the `format` argument, PIL should have no problem to decide the format based on the filename extension, this way, user can simply give names like `foobar.gif` or `foobar.jpeg` and they will be saved with correct format.

By the way, I put in `(tm)`, just push the humor to another level.