Add `chatter` command

[trezy]

User Story

As a developer, I want to generate a lot of chat traffic on a channel, So that I can load test my apps.

Acceptance Criteria

• I can send the chatter command to fdgt.
• When I send the chatter, a new chat message is sent every t seconds.
• Chat messages are sent by a varied number of users, representing user drop-in/drop-out.
• Chat message send times vary by r seconds.
• Chat messages conform to Twitch's chat message requirements.

Arguments

• t defaults to 10.
• t can be set with the --frequency flag.
• r defaults to 1.
• r can be set with the --randomness flag.

Other Notes

This is the first command that doesn't directly relate to a specific Twitch event. It should be handled be a new handler, rather than being build into the data-mocks directory.