[WIP] Added features to Decomposition.py

[naresh-bachwani]

This PR fixes #316. I added the following features :

1. Added a parameter for showing cumulative explained variance.
2. Added a parameter which takes percentage of variance needed in the components and shows corresponding principal components.

Sample Code and Plot

from yellowbrick.features.decomposition import ExplainedVariance

from sklearn.datasets import load_digits
digits = load_digits()
X = digits.data

viz = ExplainedVariance(cumulative=True, cutoff=85)
viz.fit(X)
viz.transform(X)
viz.poof("Varaince.png")

Still to do:

• Add more features.
• Add tests.
• Add documentation

More Todos:

• 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?

[bbengfort]

@naresh-bachwani I've merged these changes into #1037 so that we can incoporate them into v1.1 -- thank you so much!