Read-only mode

[netromdk]

Depends on #81 to be merged first and then rebased onto here.

Fixes #75.

Changes proposed in this pull request:

• Re-enable parsing of [] for no args instead of None
• Added read-only mode to Config that is saved/loaded with the config file
• The state can be shown and changed via config --set-read-only and config --unset-read-only
• Any instance of SlackAPI will raise an exception for any method that is destructive, modifying, or sends messages when in read-only mode
• Reset all config with config --reset when not in read-only mode and after confirming to do so

[netromdk]

Do we need to restrict other things for the read-only mode, @voidpirate?