use the PORT environment's variable if defined to start the server

### Example of use:

**Project with Choko as a dependency:**
```
PORT=3010 bin/choko start
```

**Project with Choko as a dependency, and the directory where applications are is "apps":**
```
PORT=3010 node_modules/.bin/choko start apps
```


*Just for knowledge you would get the same result by doing this:*
```
node_modules/.bin/choko start -p 3010 apps
```