Feature #2: Include PR URL in report 

Add --pr-url option to command line. Adds url after PR description.

/cc @per-nilsson

And here's an example of **this** PR with the new flag!:

```
$ ./pull-report.js -o FormidableLabs --pr-url
* FormidableLabs
  * pull-report: (1)
    * per-nilsson / ryan-roemer - 3: Feature #2: Include PR URL in report 
      https://api.github.com/repos/FormidableLabs/pull-report/pulls/3
```