feat(generate tweets)

[gwenwindflower]

Adds Tweet type to jafgen. The way this is intended to work is:

1. a random time window between 0 and 20 minutes after the order, a tweet is potentially generated
2. different Personas have different fixed tweet propensities. These are simpler than the buying propensities because they're already dependent on the more complex behavior of the buying propensities, so they're just fixed probabilities. Students and Brunch Crowd are very tweet-happy, Commuters very much not so.
3. Tweets are generated via simple rules that combine the items purchased with a randomly assigned fan_level Customers get upon instantiation, and some random selection of adjectives that correspond to the fan level (high fan levels select from positive adjectives, medium from neutral, low from negative).

This should enable some fun analysis like determining cohorts of fans via sentiment analysis.