Prepare for publishing to NPM

This PR is one step in addressing https://github.com/tildeio/router.js/issues/69. For `router.js` to exist on NPM, its dependencies should first exist on NPM. These small changes prepare this library for publishing to NPM.

* Update `package.json`:
  * Add `main` field, pointing to CommonJS build at `dist/route-recognizer.cjs.js`.
  * Update description.
  * Add version -- I chose `0.1.0` but perhaps it should be different.
* Add `.npmignore`, ignoring Ruby build files.