Display number of lines when requesting reviews via hubot

Would allow seeing at a glance how large/small a PR is. Example output:
```
@person1, @person2: please review https://github.com/imsky/pull-review/0 (81 additions, 34 deletions, 2 files)
```

Start by locating the Hubot code that formats review-request messages and the GitHub pull-request data it uses. Verify the message includes additions, deletions, and file count in the format shown; the issue names no files or tests, so identify the relevant existing coverage before changing behavior.

Written by the indexing model from the issue text.