Create script to test locally

[gemfarmer]

Eventually, it would be great to have a local cli script that tests all files in a directory using the same retext config. Until that glorious time, however, I have slightly modified retext-18f.js so that it can alternatively handle testing a test post at sample/post.html by running

node retext-18f.js ////sample/post.html

or simpler:

npm test