Add support for registering custom checks, statistics, strategies

[cosmicBboy]

Is your feature request related to a problem? Please describe.

So far, pandera has supported easily creating custom checks for dataframe/series schemas. However, with the ability to generate synthetic data #314 from a schema, it's become more important to be able to easily register checks as built-in checks so that users can:

1. serialize/deserialize user-defined checks
2. define data-generation strategies for these checks

[cosmicBboy]

fixed by #351