The `help_command` method for each addon/the bot were not returning the
help message, but were instead sending the help messages themselves.
Update the logic to simply return the help message string.
Both `!help` and `!listcommand` are handled by the bot itself, so take
advantage of how the addons method calls work to implement both
`matrix_command` and `help_command` on `self`.
Add `help_command` to each currently enabled and functional addon
With the updated means of handling addons, update Custom Commands to
include the new logic and be functional.
addons/custom_commands.rb: Update for new addons logic
