Add SlackAgent

The SlackAgent lets you receive events and send notifications to slack
Add default username
Added a liquid tag to access user credentials

Via '{% credential USER_CREDENTIAL_NAME %}' liquid enabled agent options
can be used to access stored user credentials
Merge pull request #348 from dsander/liquid-credentials

Added a liquid tag to access user credentials
Merge pull request #342 from tonmoy-saikia/slack_agent

Add SlackAgent
Merge remote-tracking branch 'origin/master' into scenarios
diagram links back, export URL includes .json
upload and fetch by url, data object manages import; next step is actual import