feat(plugins/help): add help command/plugin

Shows the help message for all other plugins or a specific plugin

Closes #4
1.3.0
feat(Bot): add custom icon support

if the `icon` key in the config starst with : then that is passed as `icon_emoji` otherwise
a url is assumed.

Closes #11
docs(README): document icon
1.4.0
feat(OutgoingMessage): setResponse() can deal with attachments

Simply pass in a hash with an attachments property

Closes #3
test(OutgoingMessage): `setResponse` with attachments
docs(README.md): Attachments
1.5.0