Add commands to modify the configuration

Description

Server admins should have a way to modify the configuration via commands. A possible implementation could look like this:

• !bbot config <key> <value>
• !bbot config message <messageText> <probability>

Acceptance Criteria

• Only server member with specified permissions / roles can modify the config via commands
• Command to change a specific config key to a new value
• Command to add a new message to the list of messages in the config