[WIP] Adds support for Pipelines with ModelVisualizers

[bbengfort]

This PR fixes #498 and adds support for Pipelines in most ScoreVisualizers. It also adds some automatic checking extending functionality described in #180.

I have made the following changes:

1. Added an is_pipeline type check
2. Added functionality to ModelVisualizer to access the final estimator in a pipeline

TODOs and questions

Still to do:

• [ ] modify visualizers to use new code
• [ ] create tests for pipelines with model visualizers
• [ ] document pipelines in visualizers

CHECKLIST

• [ ] Is the commit message formatted correctly?
• [ ] Have you noted the new functionality/bugfix in the release notes of the next release?

• [ ] Included a sample plot to visually illustrate your changes?
• [ ] Do all of your functions and methods have docstrings?
• [ ] Have you added/updated unit tests where appropriate?
• [ ] Have you updated the baseline images if necessary?
• [ ] Have you run the unit tests using pytest?
• [ ] Is your code style correct (are you using PEP8, pyflakes)?
• [ ] Have you documented your new feature/functionality in the docs?

• [ ] Have you built the docs using make html?