Add a pick function

It randomly picks from the supplied arguments based on embedded
probability counts.
Make the Markov chain more memory efficient
Add build-word-level-markov-chain-from-file
Add build-letter-level-markov-chain-from-file
Select multi-corpus start n-gram automatically

Add starting n-grams to the data structure we use to store markov
chains, so that we can pick a starting n-gram automatically according
to the starting n-gram distribution within each corpus.

Currently, we are treating each line in a file as a unique corpus.