Go-github: Support preview Lock Reason via the Lock Issue API

[gmlewis]

GitHub Developer API announcement:
https://developer.github.com/changes/2018-01-10-lock-reason-api-preview/

Supporting this change will require the following:

• [ ] add a new preview media type (application/vnd.github.sailor-v-preview+json) to github.go,
• [ ] add new active_lock_reason and lock_reason fields (ActiveLockReason and LockReason in Go, respectively) to Issue and PullRequest responses,
• [ ] add a new (optional) lock_reason field (LockReason in Go) to the IssuesService.Lock request (which will involve a breaking API change, so that the libraryVersion will need to be bumped),
• [ ] add a new lock_reason (LockReason in Go) field to IssueEvent and PullRequestEvent,
• [ ] write supporting tests, and
• [ ] bump the libraryVersion.

This is slightly more involved than some of the other recent enhancements, but would still be a good challenge for any new contributor to this repo or to a new Go developer.

All contributions are greatly appreciated. Check out our CONTRIBUTING.md document for more information on how to get started.

Feel free to volunteer for any issue, and we can send you an invite to contribute to the repo (which you then accept) and the issue can be assigned to you so that others don't attempt to duplicate the work.

Thank you!

[anirudhmurali]

Hey @gmlewis, I'm willing to work on this. Can you invite me? TIA :)

[gmlewis]

Thank you, @anirudhmurali! Invite sent.

[sandeepsukhani]

Hey @anirudhmurali, are you working on this?
If not, would you mind if I pick this up?

[anirudhmurali]

No, you can go ahead.

[sandeepsukhani]

@gmlewis
I have created a pull request for this.
Please review and let me know whether it looks good.
I can't find the code for bumping up the version. Can you please point me to that?

[gmlewis]

Thank you, @sandlis! Now that we tag this repo with semantic versioning, we no longer have a libraryVersion so there is no further action needed on your part.