p abbreviation is already used for port

Running with a different port e.g. ```-p 8090``` fails with a very strange error message:
```
path.js:313
        throw new TypeError('Arguments to path.resolve must be strings');
              ^
TypeError: Arguments to path.resolve must be strings
    at Object.exports.resolve (path.js:313:15)
    at Object.<anonymous> (/usr/local/lib/node_modules/firewatch/lib/server.js:75:28)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Function.Module.runMain (module.js:497:10)
    at startup (node.js:119:16)
    at node.js:906:3
```

This is because the ```p``` abbreviation is used both by ```--port``` as well as ```--output-path```, and the last one wins.