Change default config options
Add sensible defaults when getting configuration
Largely cosmetic changes

* Use 'is_bad' so it's clear that we can classify more than just spam
* Make sure it's floating point division and not integer division
    (It should be float division anyway, but future plugins may not respect
	this)
* Raise an instance not a class
Add docstrings for API, change `is_bad` to `match`

A positive match may be bad, as is the case for spam, or good, as is the case
for usefulness.
Handle case when configs for plugins don't exist