Add details to Slack messages

[stefanprodan]

• attach canary analysis metadata to init/start messages
• add rollback reason to failed canary messages
• add Alermanager example to alerting docs

Fix: #12

[codecov-io]

Codecov Report

Merging #14 into master will decrease coverage by 1.2%.
The diff coverage is 22.85%.

@@            Coverage Diff             @@
##           master      #14      +/-   ##
==========================================
- Coverage   45.45%   44.24%   -1.21%     
==========================================
  Files           6        6              
  Lines         880      904      +24     
==========================================
  Hits          400      400              
- Misses        425      449      +24     
  Partials       55       55

Impacted Files	Coverage Δ
pkg/controller/controller.go	6.96% <4.16%> (-1.13%)	⬇️
pkg/controller/scheduler.go	26.5% <63.63%> (-0.33%)	⬇️

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