Add option to change server port. Add help message.

* Users can now specify server port with `lua server.lua [port]`
* A help messge can be invoked by running `lua server.lua --help`

```
$ lua server.lua --help
TPTMP v1 -- Powder Toy v89.2
Usage: server.lua [port]

    Default port is 34403.

    --help        display this help message
```