Add support for linking work items in PR title/body

[joshjohanning]

In lieu of (or in addition to) enforcing work items to be linked to commits, support work items to be linked to PR title or body.

• Closes Can we do the same to validate the description of the pull request  #61 (thanks for the suggestion @bheemvennapureddy)
• Closes Add comment to PR if failing with information #63
• Closes npm install  #69
• Closes Check for prerequisite software  #70
• Closes Update JSON5 dependency #71
• Closes Improve commit message matching check #72
• Closes Support GitHub Enterprise Server #75

[joshjohanning]

There are a few things left to do, but will merge this in and get v2 deployed.