Added ability to send messages on POST request based on email
Added ability to send messages on POST request based on email

- Created a map of email and usernames
- Call to slack users.list caching the result for 1 week (by default)
the cache time can be configured in the config.yaml defining `slack_cache` in seconds (min_value: 1)

The users.list method implements cache because it's a tier2 call and there is a limit
on the number of calls and as well because the response can get very bulky.

more info: https://api.slack.com/methods/users.list

yougov/pmxbot,yougov/pmxbot,yougov/pmxbot