Twitter-markov

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* Small update for changed twitter bot utils API

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* Replace `cobe` with `markovify`. This simplifies learning, since it's just adding to a text file corpus.
* Also changes `Twitter_markov` API, removing catalyst argument and replacing 'brain' keyword arguments with 'corpus' or 'model'.
* Replace `twittermarkov learn` with `twittermarkov corpus`
* Add checking.generator function
* Restore similarity/blacklist checker for generated text
* rename class from Twitter_markov to TwitterMarkov
* Rework cli tools, renaming twittermarkov learn -> twittermarkov corpus

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* Combine two command line tools into one command with subcommands.
* Handle errors in learning more cleanly.

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* `Twitter_markov` class no longer extends Tweepy.API, so an existing API object can be passed in
* Cleaned up code around brain naming.
* Expanded readme.