Add a version and main to package.json

Currently I'm trying to `npm install` html-janitor by pointing to a github url but getting a missing version error. I tried using the version on npm but I was getting a `object is not function` error. The version on github works and is maybe [more recent than the version on npm](https://github.com/guardian/html-janitor/issues/7). This should get the ball rolling on npm support.

Amazing work with Scribe and these plugins btw! Scribe clearly stands out in it's own tier above the rest of the open source WSYWIG editors I've looked at. Truly impressive work!