Completed the basic proof-of-concept:

* added a single method to take a plaintext message and "encode" it into innocent tweets based on a dictionary and a random corpus text
* also wrote a method to take the broadcast tweet strings and "decode" it back into the original message

Example from the repl:

user> (def my-secret (tweet-secret.core/encode-plaintext "Tonight we take Paris by storm" words pg74tweets))
user> my-secret
("\"And❘ don't they wear the bulliest clothes!" "Of _co❘urse_ they do." "\"What sail's she carrying?\" \"Courses, tops'ls, and flying-jib, sir.\" \"Send the r'y❘als up!" "\"Set her back on the stabboard❘!" "Tom had be❘en restless and full of chafings and repinings;" "But it transpired that this was a request which Joe had just been going to make of T❘om, and had come to hunt him up for that purpose.")
user> (tweet-secret.core/decode-tweets words pg74tweets my-secret)
("tonight" "we" "take" "Paris" "by" "storm")

Still need to deal with cases where a plaintext token is not found in the dictionary; should also group the encoded tweets more efficiently so that multiple plaintext tokens can be represented in a single tweet, where they fit the 140 character restriction.