Detect issue links that are actually PRs

[martinemde]

Based on the pull_request key I could re-request for the PR details:

  "pull_request": {
      "url": "https://api.github.com/repos/octocat/Hello-World/pulls/1347",
      "html_url": "https://github.com/octocat/Hello-World/pull/1347",
      "diff_url": "https://github.com/octocat/Hello-World/pull/1347.diff",
      "patch_url": "https://github.com/octocat/Hello-World/pull/1347.patch"
    },

via Github Issue API docs

This would take care of PRs that are linked via the shorter syntax: #2 and hubot-slack-github-issue-link#2