Create slackv2.go for new slack Oauth endpoints

[stevengill]

Slack has introduced granular scoped apps which uses a new OAuth endpoint. When you make a new slack app, this is the new default.

The previous slack endpoint still works for classic slack apps. So I figured it might make sense to offer both still. These classic slack apps can still be made and a majority of existing slack apps use them.

If y'all think it would be better just update slack.go instead of creating a new slackv2.go, let me know. I'm a go noob, so any advice would be appreciated.

Issue: #412

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[stevengill]

@googlebot I fixed it.

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CLAs look good, thanks!

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This PR (HEAD: 92b19d6) has been imported to Gerrit for code review.

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Message from Go Bot:

Patch Set 1:

Congratulations on opening your first change. Thank you for your contribution!

Next steps:
Within the next week or so, a maintainer will review your change and provide
feedback. See https://golang.org/doc/contribute.html#review for more info and
tips to get your patch through code review.

Most changes in the Go project go through a few rounds of revision. This can be
surprising to people new to the project. The careful, iterative review process
is our way of helping mentor contributors and ensuring that their contributions
have a lasting impact.

During May-July and Nov-Jan the Go project is in a code freeze, during which
little code gets reviewed or merged. If a reviewer responds with a comment like
R=go1.11, it means that this CL will be reviewed as part of the next development
cycle. See https://golang.org/s/release for more details.

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/222781.
After addressing review feedback, remember to publish your drafts!