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.