Make error formatter configurable

[Qqwy]

The formatter itself has already been split into a separate module to allow this in the future.

However, we still need to provide a configuration setting. My current idea is to:

• Have a global setting
• Be able to override it per module
• Be able to override it per spec
• Be able to override it by passing in explicit options to conform and friends.

The other thing we'd want is to make new formatters test-able. For this, also see #3 . We have a Formatter.problem_tuple type that will (once #3 is done) contain all possible problems that might occur for all wrong typechecks, therefore being a generator of all potential inputs of the formatter.
This will make it easy to property-test custom formatters.