New option to ignore events by other workspace users in shared channels

[seratch]

Description

When this project started, Slack was not yet providing shared channels.

It's not supposed to invite Hubot users to shared channels but it may unintentionally happen. To avoid some confusion among end-users, it would be great if this project provides an option to turn off reacting to other workspace users.

The default behavior will be the same. Developers can turn off reacting to other workspace users by configuring Hubot apps.

I've already verified seratch@3190bac works as I expect in shared channels. I will come up with a pull request with a new option named INSTALLED_TEAM_ONLY.

What type of issue is this? (place an x in one of the [ ])

• bug
• enhancement (feature request)
• question
• documentation related
• testing related
• discussion

Requirements (place an x in each of the [ ])

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've searched for any related issues and avoided creating a duplicate issue.

[seratch]

I'm a bit busy with https://github.com/slackapi/python-slackclient this week and probably next week. After finishing those Python tasks, I will make a pull request addressing this issue.