Slack notification

[toricls]

I want to know about build status changes without opening GitHub. Slack notification would be very handy 😃

[toricls]

gci will allow users to configure values as follows:

• webhook url (required to use)
• channel or username to send (not required, default value is #general)
• bot's username (not required, default value is gci ($CODEBUILD_PROJECT_NAME))
• bot's icon emoji (not required, default value is :robot_face:)