More flexible KElbow colors that use default palette by default

[bbengfort]

Sets the colors in KElbow to cycle colors (C0, C1) to use the default palette rather than hardcoding color codes. Also adds properties so that users can set the metric, timing, and vline colors manually (but not passed in as parameters to init)

This PR fixes #1159

I have made the following changes:

1. Set constants for the TIMING_COLOR, METRIC_COLOR, both set to cyclic palette colors "C1" and "C0"
2. Created a colors dictionary on the visualizer and properties to set and access the colors for timing, metric, and vline
3. Updating the drawing code to use these colors
4. Created a test for changing colors
5. Updated test baseline images

Sample Code and Plot

To use the dark background palette in matplotlib:

# Import modeling 
from yellowbrick.cluster import KElbowVisualizer
from sklearn.datasets import make_blobs
from sklearn.cluster import KMeans

# Use dark background - must come after YB import
import matplotlib.pyplot as plt
plt.style.use('dark_background')

# Visualize K-Elbow
X, _ = make_blobs()
oz = KElbowVisualizer(KMeans())

oz.fit(X)
oz.show()

To set colors manually:

# Import modeling 
from yellowbrick.cluster import KElbowVisualizer
from sklearn.datasets import make_blobs
from sklearn.cluster import KMeans

# Visualize K-Elbow
X, _ = make_blobs()
oz = KElbowVisualizer(KMeans())

# Set colors
oz.metric_color = 'r'
oz.timing_color = 'c'
oz.vline_color = 'y'

oz.fit(X)
oz.show()

TODOs and questions

No TODOs, all set here!

CHECKLIST

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

• [x] Included a sample plot to visually illustrate your changes?
• [x] Do all of your functions and methods have docstrings?
• [x] Have you added/updated unit tests where appropriate?
• [x] Have you updated the baseline images if necessary?
• [x] Have you run the unit tests using pytest?
• [x] 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?

[codecov-io]

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[bbengfort]

@lwgray no idea how I got the tests to pass ... but they did; so going to merge before I blink and they fail again!