open browser when serve

About #24.  It's very useful.

And  i added an option to tell `serve` to open the browser or not. __Default is 'yes'!__

__Example:__

```
$ resume serve

Opening the browser...
Press ctrl+c to stop

/ serving....
```

This will open the browser and can preview the HTML-version of the resume.

And if you do not want to open browser defaultly, you can do like this:

```
$ resume serve -s
or
$ resume serve --silent

Preview: http://localhost:4000/
Press ctrl-c to stop

\ serving...
```

Of cause, run it with other params:

```
$ resume serve --port 80 -s
$ resume serve --port 80 --theme crisp -s
```