Added 'serve' command

Hey!

I've added the `serve` command. 
It starts a local web server that serves the HTML-version of the resume.

The HTML-version is re-generated every request. This means you can keep the server running while you edit your resume.json and then just refresh your browser to view the result.

#### Example usage:
```
$ resume serve

Preview: http://localhost:4000/
Press ctrl-c to stop

\ serving...
```

Or run it with parameters:
```
$ resume serve --port 80
$ resume serve --port 80 --theme crisp
```

#### Error handling:
`serve` also handles read and/or parse errors gracefully. For example: if you resume.json fails to parse, just fix the error and refresh your browser.