RFR: notify: add Slack notifier

[Erik Helin]

Hi all,

this is a draft pull request to add Slack support to the `NotifyBot`. @rwestberg
can you help out with implementing the `NotifyBot` part of
`handleNewPullRequest`?

Thanks,
Erik

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Commit messages:
 - Support new callback
 - notify: add Slack notifier

Changes: https://git.openjdk.java.net/skara/pull/564/files
 Webrev: https://webrevs.openjdk.java.net/skara/564/webrev.00
  Stats: 209 lines in 14 files changed: 181 ins; 16 del; 12 mod
  Patch: https://git.openjdk.java.net/skara/pull/564.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/564/head:pull/564

PR: https://git.openjdk.java.net/skara/pull/564