Support for extending existing Checks with business specific Checks

**Is your feature request related to a problem? Please describe.**
I would like to be able to add Custom Checks by only making changes in python-deequ and not in deequ (scala). As a team with no Scala knowledge, it would allow for greater efficiency and adoption.

**Describe the solution you'd like**
I'd like to be able to extend Checks with Checks specific to my business that would not be valuable to the community. For example, are the values in a column valid ISBN's?

**Additional context**
If there is a way to do this already, would you mind giving an example and I'll update the docs.

Thanks for your time.

まず既存の Checks API とそのドキュメントを読み、Scala Deequ レイヤーを変更せずに Python がすでに拡張経路を公開しているかを確認します。完了条件は、ISBN 検証のような business check について Python のみで記述した例をドキュメント化するか、そのような check をサポートする範囲を限定した実装を行うことです。

索引モデルが issue の本文から書いたものです。