This class was design only for working with SimpleWriter, so nesting it makes
this clear.

This class used to know too much about the app-specific needs.
It's now a thin wrapper around the Slack API client gem.

A major change though, is that it automatically fetches the message
author real name, so it issues an additional API call for each message.
To remedy the performance hits, all the author names are requested in
parallel, with a cache to avoid unnecessary requests.

This class is responsible for "exploring" Slack and "mining" interesting
messages. Those messages, later become a blog post.

Now that we have a simple Slack client, we can put the business logic
in the explorer. This also opens the door for more explorers in the future,
like a Twitter explorer.