Add slack support

• Add slack notification support to Lighter deploys (#58)

0.20.0

• Support posting custom release notes during the hipchat notification (#55)

0.19.1

• Improved error reporting (#54, #52)

0.19.0

• Support canary deployments (#47)

0.18.1

• Preserve variable types (#51)
• Fix problem with profile importing (#51)

0.18.0

• Added profile support for including extra yaml files at runtime (#50)

0.17.1

• Fixed Graphite event tagging to account for incorrect Graphite API documentation

0.17.0

• Support Graphite deployment events (#48)

0.16.2

Switched datadog lighter.deployment metric to datadog.events metric because we wish to sum different series
Added datadog status tag to filter on
Changed datadog.events metric to a count since gauge doesnt fit the purpose

0.16.1

Avoid aggregating Datadog deployments across environments
Added standard Datadog tag 'type:change' to improve filtering