Slack notifier implementation

[joulei]

Description

Adds main functionality for the slack notifier

Notes for reviewers

The notifier implementation parts from Boom version 0.8.0 which hasn't been released yet.
In order to run tests, a local version of boom 0.8.0 must be set up, e.g. {:boom_notifier, path: "../boom"},

Additionally, to run locally, you'll have to specify a slack_webhook_url as described in the readme, instructions are also provided for creating a new slack application and setting up a webhook.

Screenshots

[joulei]

Squashed newer commits for clarity,
Had to switch back to Mix.config module instead of Config as it was just introduced in Elixir v1.9.
Similar changes for stack trace, as it's being formatted differently across elixir versions.

[joulei]

As discussed with @mcass19, I replaced the BoomSlackNotifier.SlackClient.HttpAdapter behavior return types with more generic ones, so that users can implement it around their desired HTTP clients.
Added a section on the readme and a moduledoc to document it.

[mcass19]

Great job 🚀 🚀

Leave some minor things. Some of them are more opinionated syntactic things, so feel free to ignore all what you want

[iaguirre88]

🚀🚀🚀