Transition to using YAML for configs

[Gorialis]

This is a change I've wanted to do for some time.

Pros

• YAML looks nice
  • Scalar flow-sequences are more human-readable than direct lists
  • Folded scalars could be useful for extended string content (see Custom Messages #30)
• YAML's inherent dict state allows values to be combined on top of a default (as in this implementation), allowing configs to be shorter as not every value needs to be defined
• No need for awkward hasattr and getattr situations

Cons

• Existing configurations must be changed
• YAML cannot define functions or such which reduces the dynamics of the config
• Requires extra dependency PyYAML
• As the config is attached to the bot instance, it increases the 'uncleanliness' of code where the bot is not easily accessible

I've tested this impl a bunch and haven't found anything that's broken, and after checking through all changes and doing some tests I think this is 'complete'. The only thing that might change is the behavior of Configuration.__getattr__ to not return None when something is missing (raising instead).

[slice]

consider it approved for me