It would be nice if the mdl serve livereload port were specifiable or the same as the -port port

I noticed while trying to run a second instance of `mdl serve` with a different port number, that it still failed due to trying to bind to port `35728`:

```text
listen tcp :35729: bind: address already in use
```

It seems like the code is currently always passing the default port from the `github.com/jaschaephraim/lrserver` package when starting it, so it is definitely possible to pass a different value as a parameter.

It would probably be a little more work to get a live reload handler into the main `mdl serve` server since the `github.com/jaschaephraim/lrserver` package doesn't currently provide an exported handler method that could be used, but that would mean that you only need to pass one parameter in order to use multiple instances of `mdl serve` concurrently.