Spec.conform_valid can now propogate through nested Specs

[chrisrink10]

This PR adds an IConformer interface for conformers internally. That interface allows internal custom conformers to propogate calls to e.g. Spec.conform_valid through other nested Spec conformers. User provided conformers can still be provided as single-argument functions -- they will be wrapped automatically by dataspec into an implementation of IConformer.

I considered going the slightly different route of having dataspec wrap user functions in another function which discarded the is_valid keyword argument, provided the user function did not include the is_valid keyword argument. There's no particularly strong philosophical argument against doing this (and it is my preferred option), but it feels too unreliable to use something like inspect at runtime to figure out the signature of a function and dynamically wrap it. We already do this for determining whether to create a PredicateSpec versus a ValidatorSpec and it feels dirty enough.

Fixes #51

MyPy's plugin for attrs had a lot of trouble with the types used in this changeset. In particular, make_conformer is used as the converter for all builtin conformer attributes and for some reason MyPy simply couldn't handle that. I briefly surveyed some related issues on MyPy's repo and it seems perhaps the attrs plugin takes some shortcuts for optionals and generics, which may explain our problem.