Text Visualizations about yellowbrick

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Think about how we visualize an entire corpus versus a specific document. In the base class, perhaps the default is to visualize the full corpus, but if you pass in a specific document, you can see the stats for that document.

[rebeccabilbro]

reminder to add this to the documentation, too:

• in api/ create yellowbrick.text.rst
• in api/ add yellowbrick.text to the modules.rst toc tree
• in examples/ add text.rst to demonstrate examples of text visualizers in action

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also tests

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Preliminary text module and TextVisualizer base class implemented. Will start working on examples for the documentation and tests once we have worked on #106 and #107.