add an optional 'message' parameter to the 'stop' method

Sometimes, the message to be displayed when stopping the spinner is not known until the end (eg to display some result, elapsed time, ...). For these cases, an optional `message` parameter is added to the `stop` method. If specified, this message overwrites the default `doneMessage` from the `Linespin` constructor.
It also makes the `stop` method consistent with the `error` and `warn` methods.