Commandline port

Added the option of passing a port number when starting up the app, for the edge cases where PORT isn't set in your exports and port number 3000 is being used for something else. 

I find it a nice convenience to be able to specify a port number when booting up a project I haven't played with before. 

Useage: 
```node app.js -p 9000```

Some things that came to mind when applying this: 
a) I don't find it fully necessary to add optimist for this single case, but I let it slip through given that someone might want to add it further down the line for some other cli arguments. Problem solved beforehand. 

b) Dependency specific, is it 'valid' or rather, acceptable - to pass "latest" as the version number? Would you prefer a given number over "latest"?