Rearrange core and example module locations

[richteer]

Core modules for Halibot should be supplied with an install, and should contain only modules that would most likely be useful to all/most installs by default. Furthermore, documented example modules should not live in the source tree for Halibot, and instead should probably exist in their own repository. Since these modules need not be actually useful, they shouldn't be part of the default Halibot install either.

Proposing the following:

• Move Hello to a new repo, Halibot/Examples
• Eventually split CLI (see Split main.py into core modules #102 ) into Halibot/halibot/packages/core/*.py

Packages in Halibot/Examples should be well-commented reference modules, agents, etc that exercise supported Halibot features, and should be kept in sync with Halibot-core changes (we may want to integrate tests?).

[richteer]

Piggy backing on this issue to brainstorm other examples we may want to include:

• HalFilter examples: dropping messages, message modification
• CommandModule example
• HalAgent example (somehow)