add helper method so autocomplete can replace with attributed strings

[eliburke]

• 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 been mindful about doing atomic commits, adding documentation to my changes, not refactoring too much.
• I've added a descriptive title and added any useful information for the reviewer. Where appropriate, I've attached a screenshot and/or screencast (gif preferrably).
• I've listed my changes on the [Changelog(https://github.com/slackhq/SlackTextViewController/blob/master/CHANGELOG.md) file.
• I've read, agree to, and signed the Contributor License Agreement (CLA).

PR Summary

Adds a helper for autocomplete to replace with attributed strings

This allows richer treatment of, e.g. mentions.