Enhancement: bot framework can send DMs

[MarkAckert]

The bot framework should be able to send direct messages to users with a simple API call.

Some use cases require the bot framework to be able to send direct messages which are context-switching. For instance, when information as part of a response to a query is subject to security or privacy consideration, e.g., a user may enter a command in a public channel which surfaces protected information, in which case the bot must be able to respond via direct message to prevent that information from reaching a public channel where the command was originally posted.

Any implementation details for detecting privacy-protected information is left to callers of this framework.

[MarkAckert]

Worked on in #17