Release 0.5.0

[Gerrrr]

Release notes:

MAIN IMPROVEMENTS

• Specific change-threshold for node/service/check (Support change-threshold for node/service/check, closes #69 #186)
• Better format for multi-line messages (Use better format for multi-line messages #185)
• Notifications are not triggered if the new healthcheck is passing (do not trigger a notification if the new healthcheck is passing #169)
• Number of calls to KV is reduced when checking against blacklist (Improve checking against blacklist by reducing the number of calls to KV #167)
• Number of unnecessary writes (PUT requests) to KV is reduced when there is no change in health check status (Reduce unnecessary writes (PUT requests) to KV when no change in health check status #166)
• Support for mattermost notifications (Add notifications support for mattermost #165)
• Variable overrides and profile selection based on regular expressions (Variable overrides in the notification profile and profile selection based on regular expressions #164)
• ACL token is not shown in the logs (Drop ACL token from logs #160)
• Backtips are not prined for empty slack messages (avoid print out backticks with empty slack message #151)

and numerous bug fixes.

I hope I did not forget anything.

@fusiondog Can I kindly ask you to review and merge it after #188 and then cut the release?
Thank you in advance.