add CLI flag for custom server port

Currently, the server always runs on the same port (arbitrarily chosen to be 9345). This should be customizable by the user. I'd like us to add a command line option (maybe `-p` and `--port`) so that the user can specify a port.

We could probably bring in [structopt](https://crates.io/crates/structopt) to parse command line options.

We should keep a default port number in case the user doesn't specify a custom port.

## Example Use

```shell
$ cargo gui --port 7777
```

```shell
$ cargo gui -p 7777
```