Topics/server command

It would be amazing to be able to start the server with a shell command.  
I know you can write a module and it is a more flexible approach but the common use scenario is simpler.

Usually you just want to start a server to process some back-end language, a php form or something similar, this way I added the ``command`` property to the ``server`` key in the ``config.coffee``

You can type something like:
```
server:
  command: 'php -S 0.0.0.0:3000 -t public'
```

Hope you enjoy it =]
When the config changes or new packages/plugins are installed.

https://github.com/lucasdupin/brunch/blob/topics/server-command/src/watch.coffee#L291-L307