Thread-based conversational support

[Shadowfiend]

Add a middleware or similar that supports conversation context based on thread id. Could be better-implemented on the underlying hubot code.

This would allow commands that require a back-and-forth. Context should probably be evicted after a certain amount of time, maybe with a tombstone marker so the bot can let the user know that the context has been forgotten and to start the conversation over.

One possibility here is for “commands” that may require input from multiple people. Thread-based context would allow that input to come in via the same thread, and accumulate until the necessary info is available to execute a command. For example, if we wanted inviting someone to GitHub to require them to provide additional info (e.g. their keybase), someone might request inviting that person to GitHub, then Valkyrie might ask the new person to provide their keybase to verify access, then once that info is provided run the original request.